11.3-U2.1 is a hotfix release of 11.3-U2 which only addresses a critical issue when exporting and destroying pools (NAS-105782 Link).
FreeNAS version 11.3-U2 has officially been released. This version nearly includes a combined 150 bug fixes, updates, and improvements. Some highlights of this version include:
An update to Samba, version 4.10.13 (NAS-105349)
Bug fix when importing a pool (NAS-105297)
Fix for a middleware memory leak (NAS-104437)
Mitigation for specific LSI 9X00 cards (NAS-105568 )
For a complete, detailed list of updates, see the list of FreeNAS 11.3-U2 Jira tickets.