So first, look into the transport entity Data\attrib\ebps\races\space_marines\troops\rhino.rgd (for example), for the squad_hold_ext extension. Therein, as acceptable_type_01, you will see the type_transportable\transport_infantry.lua type. Other tranports may have acceptable_type_02, acceptable_type_03, etc (you can add more if you like yourself manually).
So these are the transportation types each transport can carry.
Now go to look into a squad, say Data\attrib\sbps\races\space_marines\space_marine_squad_tactical.rgd. In the space marines squad, you will see the same transport_infantry type in the squad_transportable_ext, meaning that the Rhino can transport them.
So in order to make a squad transportable by a specific transport, just look into the transport's squad_hold_ext, and use one of the acceptable_type_01, _02, etc, for the squad in question as well, in its squad_transportable_ext.
NOTE 1: The activate_transport_ability boolean setting of the squad, is associated with modifiers_no_squad / modifiers_squad_01, etc, of the transport entity (look into the of the squad_hold_ext of the Rhino). If you have set it to true for the space marines, then once embarked, they will enable the modifiers that follow. For one squad, the modifiers_squad_01, for two embarked squads the modifiers_squad_02, etc.
This is for example for enabling the on-transport weapons, once a squad is inside. But it is a bit advanced, as it involves hardpoints.
NOTE 2: Not only transport entities can use the squad_hold_ext, but also structures. See for example the Orbital Relay of the Space marines.Go to the full post