Use OMAPI for anything app-specific. It is a versioned, app-owned REST surface backed by a JavaScript script; the Passport Service gates access; every call is logged with input, output, status, and timing for 30 days. External systems calling your app should hit OMAPI endpoints, not the Item Service directly. From 6.0, OMAPI endpoints will adopt OpenAPI specifications for client generation.Use direct REST calls to a back-end service when the external system genuinely wants the raw platform resource (a data ingestion tool loading items in bulk, an automation writing files). Wrap the call in an OMAPI endpoint if the semantics are app-specific.