The external system ticket is generated automatically when Service Desk Manager (SDM), Service Catalog (SC) and Unified Self Service (USS) are integrated. In particular is created in Service Catalog, the ID is hard-coded and cannot be modified out of the box, it consists of three parts:
The first alphabetic part refers to the application where it came from "CASC" meaning CA Service Catalog.
The second is an internal ID number generated in CA Service Catalog.
The third one is the ID of the ticket created in USS which can also be seen in Service Catalog.