In CA-TPX you can start sessions also to applications not defined in the Application Characteristics Table (ACT) by e.g. the '/A applid' command. CA-TPX always assigns a unique virtual terminal (UNQ) for these sessions. TPX has to use a UNQ one here because Ca-TPX doesn't know about how the application behaves, e.g. does the application do a CLSDST Pass etc.
If you want to use a different type of virtual terminal then you need to add the Application to the ACT where you can specify an appropriate type of virtual terminal like SHR or GRP in the Application Characteristics Table.