The PT (Priority / Type), Trgt (Target), and Src (Source) fields define the first 3 bytes of a message in the Messages Editor for these networks:
*Note: Use the Tools/Options/Vehicle Networks tab, J1850 VPW Mode selector to switch between the 1 byte (Chrysler) and 3 byte (GM Class 2) headers. If a 1 byte header is selected, then the first 3 bytes will be ID, B1, and B2.