Inter RF Subsystem Interface (ISSI) link allows users of certain talkgroups to automatically roam onto other P25 systems.  A temporary talkgroup is automatically assigned and linked back to the user's home talkgroup on the P25 system. The talkgroups are part of a pool that are automatically randomly assigned for these temporary ISSI links on an as-needed basis, with no user, dispatcher or system administrator intervention required.

As mentioned Patch talkgroups are not permanent, and change.  Talkgroups patched through: (Encrypted if the original TG is already encrypted)