Safe-send-by is the advisory projection in a send plan that compares your deadline with the time needed to deliver an audience at a safe rate. It can recommend split or move_deadline, but it does not hold, defer, or spread sends automatically.
For example, if an audience cannot fit within the available per-ISP headroom by Friday, the projection can recommend splitting it. You decide whether to change the audience or deadline.
See SEND-12, Safe-send-by, pacing, and “split or move the deadline”.