A trim schedule of weekly or monthly with no trim-day or trim-time is one refused value naming it; a keep count of 0 is that kind's none: the existing scheduled ZFS snapshots are kept and a warning names them.
Builds: design 42 zfs-owns-zfs §20, §21, §22, A-33, A-35
A value of another form is refused before anything is written or run; absent and empty are not managed.
Builds: design 42 zfs-owns-zfs §9, §11, A-15, A-23
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