Sub-type of the service. The enum is shared by all services, but each service accepts only its own values. Required when service=BOT (gpt|messages): a value outside that pair (for example llm) is not accepted and the response carries an error message instead of a sum, because BOT is read from two distinct indices and has no default to fall back to. Optional when service=AGENT (llm|tts|stt) to narrow the sum to a single AI-agent service; omit to sum across LLM, TTS and STT. For service=AGENT (and for the services that do not use subType at all) a value that does not belong to the requested service (for example gpt when service=AGENT) is IGNORED: the response is 200 and contains the full sum across all sub-types, exactly as if subType was omitted. This does not apply to service=BOT, which still rejects a foreign value as described above.
