What you may send, and when
Holding a Page token does not mean you may message people freely. Messenger restricts sending by time since the customer last wrote, and the restriction is the platform's, not ours. It is the rule most likely to surprise someone who has used a broadcast-shaped channel before.
The 24-hour window
Within 24 hours of a person's last message, the Page may reply freely. This covers almost everything support and sales actually does: someone asks what a class costs, you answer.
Past 24 hours, an untagged send is refused. The message does not queue and does not arrive late; it is rejected.
Tags, and the one that fits
Outside the window a send must carry a tag declaring why the interruption is legitimate. The tag that fits a person answering a question is HUMAN_AGENT, which extends the window to 7 days for exactly that case: a human, following up on something the customer raised.
The others are narrower and mean what they say — an account update, a confirmed event, a post-purchase update. A tag is a claim about the content, and using one that does not describe the message is a policy problem rather than a technical one.
What this rules out
You cannot use this to broadcast. There is no tag for "we have a promotion on", and reaching people who have not written recently is a paid-messaging product rather than an API call. If a campaign is what you want, this is the wrong tool and the refusal will be immediate rather than subtle.
Practically: this channel answers, it does not announce.
Replying alongside another tool
Unlike a LINE channel, whose single webhook slot one integration must own, a Facebook Page can have several apps connected at once. Connecting here does not displace an inbox tool a team is already using, which makes the migration safe to do gradually.
It also makes double-replying possible. Two systems answering the same customer is worse than a slow answer, so while another tool still owns the replies, read here and send there. Decide which system owns the reply before turning sending on, not after the first duplicate.
Who a message is addressed to
Sending needs the person's page-scoped id, which comes from a conversation's participants rather than from anything a person would recognise. The same human has a different id on every Page they talk to, by design: an id from one Page means nothing on another, and there is no cross-referencing them.