diff --git a/lib/bookkeeping/__tests__/engine-dimension-validation.test.ts b/lib/bookkeeping/__tests__/engine-dimension-validation.test.ts
index 98175b34..4308a57c 100644
--- a/lib/bookkeeping/__tests__/engine-dimension-validation.test.ts
+++ b/lib/bookkeeping/__tests__/engine-dimension-validation.test.ts
@@ -390,4 +390,63 @@ describe('commitEntry — mandatory dimension enforcement (PR10)', () => {
 
     expect(queriedTables()).not.toContain('journal_entry_lines')
   })
+
+  it('exempts system source types — an untagged SIE-import entry commits despite a required rule', async () => {
+    const { supabase } = buildSupabase({
+      ...BASE_TABLES,
+      account_dimension_rules: { data: [requiredRule] },
+      journal_entry_lines: {
+        data: [
+          // Post-cutover line fetch carries the parent source_type via join.
+          { account_number: '4010', dimensions: {}, journal_entries: { source_type: 'import' } },
+          { account_number: '1930', dimensions: {}, journal_entries: { source_type: 'import' } },
+        ],
+      },
+    })
+
+    const entry = await commitEntry(supabase as never, 'company-1', 'user-1', 'entry-1')
+
+    expect(entry.id).toBe('entry-1')
+    expect(supabase.rpc).toHaveBeenCalledWith(
+      'commit_journal_entry',
+      expect.objectContaining({ p_entry_id: 'entry-1' })
+    )
+  })
+
+  it('still enforces on operational source types carried by the join', async () => {
+    const { supabase } = buildSupabase({
+      ...BASE_TABLES,
+      account_dimension_rules: { data: [requiredRule] },
+      journal_entry_lines: {
+        data: [
+          { account_number: '4010', dimensions: {}, journal_entries: { source_type: 'manual' } },
+        ],
+      },
+    })
+
+    await expect(
+      commitEntry(supabase as never, 'company-1', 'user-1', 'entry-1')
+    ).rejects.toBeInstanceOf(MandatoryDimensionMissingError)
+  })
+})
+
+describe('createDraftEntry — system-source exemption (PR10)', () => {
+  it('never applies default/fixed rules onto an opening-balance entry (no injection into derived history)', async () => {
+    const { supabase, inserts, queriedTables } = buildSupabase({
+      ...BASE_TABLES,
+      account_dimension_rules: {
+        data: [makeRuleRow({ rule_type: 'fixed', dimension_values: { code: 'PLOCK' } })],
+      },
+    })
+
+    const input = { ...makeInput(), source_type: 'opening_balance' as const }
+    const entry = await createDraftEntry(supabase as never, 'company-1', 'user-1', input)
+
+    expect(entry.id).toBe('entry-1')
+    // Exempt source: the rules table is never even consulted…
+    expect(queriedTables()).not.toContain('account_dimension_rules')
+    // …and the inserted bags stay exactly as the import provided them.
+    const lineRows = inserts.journal_entry_lines[0] as Array<Record<string, unknown>>
+    expect(lineRows[0].dimensions).toEqual({})
+  })
 })
diff --git a/lib/bookkeeping/dimension-rules.ts b/lib/bookkeeping/dimension-rules.ts
index 65101ac0..9e8ea62c 100644
--- a/lib/bookkeeping/dimension-rules.ts
+++ b/lib/bookkeeping/dimension-rules.ts
@@ -174,3 +174,37 @@ export function assertMandatoryDimensions(
     throw new MandatoryDimensionMissingError([...violations.values()])
   }
 }
+
+/**
+ * Source types EXEMPT from dimension rules — system-generated and
+ * correction-instrument entries where policy must never bite:
+ *
+ *   - historical/derived data (SIE import, opening balances) must land
+ *     verbatim — injecting defaults or refusing untagged history would
+ *     falsify the record (BFL 5 kap)
+ *   - year-end and revaluation are system bokslut mechanics; a rule on a
+ *     result account must not be able to block closing the year
+ *   - storno/correction/credit notes are HOW history gets fixed — blocking
+ *     them on entries that pre-date a rule would make old mistakes
+ *     permanent (same argument as the commitEntry bypass for reversals)
+ *   - accrual dissolutions replay a schedule created before the rule
+ *
+ * Operational sources (manual, bank_transaction, invoice_*, supplier_*
+ * registrations/payments, salary_payment) stay enforced — those are the
+ * new business events the policy exists for.
+ */
+export const DIMENSION_RULE_EXEMPT_SOURCE_TYPES: ReadonlySet<string> = new Set([
+  'opening_balance',
+  'import',
+  'year_end',
+  'storno',
+  'correction',
+  'credit_note',
+  'supplier_credit_note',
+  'currency_revaluation',
+  'system',
+])
+
+export function isDimensionRuleExemptSource(sourceType: string | null | undefined): boolean {
+  return sourceType != null && DIMENSION_RULE_EXEMPT_SOURCE_TYPES.has(sourceType)
+}
diff --git a/lib/bookkeeping/engine.ts b/lib/bookkeeping/engine.ts
index c8e2641a..5bd3d883 100644
--- a/lib/bookkeeping/engine.ts
+++ b/lib/bookkeeping/engine.ts
@@ -22,6 +22,7 @@ import {
   applyDimensionRules,
   assertMandatoryDimensions,
   fetchActiveDimensionRules,
+  isDimensionRuleExemptSource,
 } from '@/lib/bookkeeping/dimension-rules'
 import { backfillStandardBASAccounts } from '@/lib/bookkeeping/account-backfill'
 import { syncInvoiceStatusFromPaymentEntry, isPaymentSourceType } from '@/lib/bookkeeping/payment-sync'
@@ -235,8 +236,11 @@ export async function createDraftEntry(
   // Account dimension rules (dimensions PR10): apply 'default'/'fixed'
   // values onto the line bags before validation + insert. Zero rules —
   // every company by default — returns the input untouched; a failed rule
-  // fetch fails open like the soft validation below.
-  const rules = await fetchActiveDimensionRules(supabase, companyId)
+  // fetch fails open like the soft validation below. System-generated and
+  // correction sources are exempt — policy governs new business events,
+  // never imported history or bokslut mechanics.
+  const ruleExempt = isDimensionRuleExemptSource(input.source_type)
+  const rules = ruleExempt ? [] : await fetchActiveDimensionRules(supabase, companyId)
   if (rules === null) {
     log.warn('dimension rule fetch failed — defaults/fixed skipped (fail-open)', { companyId })
   }
@@ -424,8 +428,13 @@ export async function updateDraftEntry(
 
   // Same soft dimension validation as createDraftEntry — before any write, so
   // a rejection leaves both the header and the existing lines untouched.
-  // Account dimension rules (PR10) apply first — same as create.
-  const rules = await fetchActiveDimensionRules(supabase, companyId)
+  // Account dimension rules (PR10) apply first — same as create. Gate on
+  // the STORED source_type (updates preserve it; the input's copy is not
+  // authoritative here).
+  const ruleExempt = isDimensionRuleExemptSource(
+    (existing as { source_type?: string }).source_type
+  )
+  const rules = ruleExempt ? [] : await fetchActiveDimensionRules(supabase, companyId)
   if (rules === null) {
     log.warn('dimension rule fetch failed — defaults/fixed skipped (fail-open)', { companyId })
   }
@@ -567,7 +576,7 @@ export async function commitEntry(
   } else if (rules.some((r) => r.rule_type === 'required')) {
     const { data: ruleLines, error: ruleLinesError } = await supabase
       .from('journal_entry_lines')
-      .select('account_number, dimensions')
+      .select('account_number, dimensions, journal_entries!inner(source_type)')
       .eq('journal_entry_id', entryId)
     if (ruleLinesError || !ruleLines) {
       log.warn('line fetch for mandatory dimension check failed — enforcement skipped (fail-open)', {
@@ -575,10 +584,17 @@ export async function commitEntry(
         entityId: entryId,
       })
     } else {
-      assertMandatoryDimensions(
-        ruleLines as Array<{ account_number: string; dimensions: Record<string, string> }>,
-        rules
-      )
+      const typedLines = ruleLines as unknown as Array<{
+        account_number: string
+        dimensions: Record<string, string>
+        journal_entries: { source_type: string }
+      }>
+      // System/correction sources are exempt — see
+      // DIMENSION_RULE_EXEMPT_SOURCE_TYPES (imported history, bokslut
+      // mechanics and credit instruments must never be blocked by policy).
+      if (!isDimensionRuleExemptSource(typedLines[0]?.journal_entries?.source_type)) {
+        assertMandatoryDimensions(typedLines, rules)
+      }
     }
   }
 
diff --git a/lib/import/__tests__/sie-dimensions-roundtrip.test.ts b/lib/import/__tests__/sie-dimensions-roundtrip.test.ts
index 488629ab..ea720774 100644
--- a/lib/import/__tests__/sie-dimensions-roundtrip.test.ts
+++ b/lib/import/__tests__/sie-dimensions-roundtrip.test.ts
@@ -54,14 +54,26 @@ const SOURCE_SIE = [
   '#DIM 1 "Kostnadsställe"',
   '#DIM 6 "Projekt"',
   '#UNDERDIM 2 "Kostnadsbärare" 1',
+  // Custom dimension + custom child — exactly what PR10's "Ny dimension"
+  // (SIE 20+, optional parent) produces. Proves the round-trip covers
+  // user-created dims, not just the reserved 1/2/6 set.
+  '#DIM 20 "Avdelning"',
+  '#UNDERDIM 25 "Team" 20',
   '#OBJEKT 1 "KS01" "Butiken"',
   '#OBJEKT 2 "KB1" "Bärare ett"',
   '#OBJEKT 6 "P001" "Villa Almgren"',
+  '#OBJEKT 20 "SYD" "Avdelning Syd"',
+  '#OBJEKT 25 "T1" "Team ett"',
   '#VER A 1 20260115 "Hyra januari"',
   '{',
   '#TRANS 5010 {1 "KS01" 2 "KB1" 6 "P001"} 15000.00',
   '#TRANS 1930 {} -15000.00',
   '}',
+  '#VER A 3 20260117 "Avdelningskostnad"',
+  '{',
+  '#TRANS 5010 {20 "SYD" 25 "T1"} 800.00',
+  '#TRANS 1930 {} -800.00',
+  '}',
   '#VER A 2 20260116 "Odeklarerat projekt"',
   '{',
   // P002 is referenced but never declared via #OBJEKT — import synthesizes