diff --git a/BACKLOG.md b/BACKLOG.md
index a5b2c89..998299b 100644
--- a/BACKLOG.md
+++ b/BACKLOG.md
@@ -277,10 +277,16 @@ Split into independently deployable sub-slices (CLAUDE.md §13):
 
 ## Iteration 4 — Objecten and the authoritative register *(milestone: `Iteration 4 — Objecten`)*
 
-### S-18 · Objecten + Objecttypen up in compose; Register objecttype defined
+### S-18 · Objecten + Objecttypen up in compose; Register objecttype defined *(split — #19 closed)*
 
 **Outcome:** Objecten and Objecttypen running. A `RegisterRecord` objecttype defined with the public-safe schema.
 
+Split into independently deployable sub-slices (CLAUDE.md §13):
+
+- **S-18a** (#139) · Objecttypen API up in compose (own DB + seeded config + health + static token).
+- **S-18b** (#140) · Objecten API up in compose, wired to Objecttypen. Depends on S-18a.
+- **S-18c** (#141) · RegisterRecord objecttype defined + registered (public-safe JSON schema). Depends on S-18a/b.
+
 ### S-19 · ACL extension: write register-record to Objecten on approval
 
 **Outcome:** Approval path writes the canonical register record to Objecten, not OpenZaak eigenschappen. Projection now sourced from Objecten events.