Use a proactive sendType when you want to send a message campaign to users without them having sent a message beforehand.
A campaign with proactive sendType can be sent through all sending endpoints and integrations.
Use a reactive sendType when you want to send a message campaign to users after they have sent a message beforehand.
The reactive sendType only allows sending a campaign through an integration (e.g., AutoReply way) but not through the sending endpoints.
In both cases, there may be limitations on certain message formats depending on the channel or channels through which the campaign will be sent.