The soft cap and hard ceiling are per-recipient frequency thresholds over a rolling 30 days. The soft cap coaches without blocking; the hard ceiling denies the message. Around 4 and 8 are configurable defaults, not promises.
For example, if a recipient reaches a configured hard ceiling of 8, the denial reason names the count and ends with (ceiling 8). Other recipients can still proceed.
See SEND-6, Frequency protection: soft cap and hard ceiling.