For this discussion, users are dynamic and SMRT Profile Selection is PROF.
How is the TPX ACT (Application Characteristics Table) related to a user's TPX profile or menu?
TPX for z/OS
The TPX Menu is made up of sessions.
When a dynamic user signs on to TPX and the SMRT profile selection method is PROF, TPX will make a security call for each profile in Profile Maintenance. When a positive response is received by TPX from external security, the sessions within that profile are added to the user's TPX menu. This continues for each profile, building the menu for that user. So the user's TPX Menu can be built from multiple TPX profiles.
System level parameters are defined within the ACT. Note that there are some parameters that are only available to be defined within the ACT such as terminal Type (GRP, SHR, UNQ). If there is no ACT entry for an applid defined at the profile (or user) level, then Type will default to UNQ (unique) and there is no way to override that without an ACT entry. It is a best practice to define ACT entries.