Required unless content_mode=ai.

static sends the template or content_html you provide. ai has the AI write the message for every send from ai_instruction; template_id and content_html are ignored, and subject is optional (leave it blank and the AI writes one; set it and it is used exactly). An ongoing or recurring campaign gets a fresh message each run, and each message is kept and listed at GET /api/campaigns/{id}/messages.

continuous stays active until stopped and sends once to each current or future subscribed contact (no repeats). recurring stays active until stopped and emails the WHOLE audience on every scheduled run; the audience is read at run time, so a contact added after a run receives nothing until the next run, then joins it. For both, action=send starts it and scheduled_at sets the earliest start.