oi_151a_prestable5 AKA oi_151a5 is a bug and security fix release. Yes, that says prestable5, having the prestable number different from the a* number needlessly confused people so hopefully we’re all clear now.
This is an ISO release.
This release includes the async zfs destroy feature from illumos that comes with a new zpool version.
Remember upgrading a zpool is a non-reversable command and any zpools you upgrade may not be readable with previous versions of OI, including other BEs on the same system.
The ZFS feature flags concept is documented here: http://blog.delphix.com/csiden/files/2012/01/ZFS_Feature_Flags.pdf. In this context the “zpool version” becomes a legacy concept, and the number is set to 5000 on existing pools during a “zpool upgrade” run.
See Release Notes.