TBASIC is effectively controlled by the Ladder Logic. So you basically have a bunch of BASIC subroutines that are called from the ladder and can call each other.
It's not an either/or situation, rather TBASIC is used to perform data processing when the right logic becomes true. Eg: Checking voltage levels on the ADCs, performing math and string handling, issuing motion commands, updating the LCD, managing slaved communication devices and plenty more.
Evan