A set is a named group of models that run together. Sets live at the top of the uint8 range, not just above the registry:

Constant	Value	Members
AI_PARALLEL_MIC	255	every microphone model
AI_PARALLEL_ROOM	254	radar plus the room-suited microphone models
AI_PARALLEL_INTRUDER	253	motion, radar and the alarm models (contains an IMU model — raises the CM33 sensor push rate)
AI_PARALLEL_ALL	252	every registered model — the widest watch
AI_PARALLEL_FIRST	= AI_PARALLEL_ALL	anything at or above this is a set, not a model

The older spellings 13 / 14 / 15 (INTRUDER / ROOM / MIC) still work: the firmware translates them, and ai_engine_active() answers with the value in the table. So edge_ai.select(13) followed by active() == 253 is CORRECT, not a mismatch. The MicroPython side and the CM55 side translate with the same model_link_resolve_set() from ipc_model_link_defs.h, and ai_engine.h static-asserts that its constants match that definition. Sets are available only in an image built with at least one microphone model.

Two hard facts about sets:

AI_PARALLEL_ALL (252) cannot be redefined — ai_engine_set_define() returns -1 for it unconditionally; only INTRUDER, ROOM and MIC can be.
ai_engine_snapshot() is not valid inside a set: it returns the most recent verdict from any model, so three quiet members bury a detection before it is read. The detection is not late, it is gone. Use ai_engine_snapshot_model() per member.