There are quite a few new features in Ops Center 12.3.1, but I think that the group of zones improvements are worth a particular mention.
The biggest is expanded support for kernel zones. In 12.3, you could discover kernel zones, but you couldn't create them. Once you've upgraded to 12.3.1, though, you gain the ability to create kernel zones, to edit existing kernel zones, and to migrate logical domains that host kernel zones. Since kernel zones are more independent from their host than other zones, kernel zones also have a storage tab that's similar to that for logical domains.
There are also some improvements for existing zones. On zones using Oracle Solaris 11.2 or later, you can dynamically edit network, file system, and storage attributes without having to reboot the zone.
The Virtualization Reference guide has more information about how to use these features, and the What's New will tell you about the other new features in 12.3.1.
EDIT: An earlier version of this post incorrectly stated that kernel zones could be migrated directly. Logical domains that host kernel zones can be migrated, but kernel zones cannot yet be migrated directly.