If you're on Size Matters, then it's a matter of the transport volume. Different units take up different amounts of volume on transports and the transports have a volume capacity. What types they can transport is a little looser... there's no distinction between 'people' and 'troops' but there is a distinction between aircraft and troops and missiles. Without being on SM, it is all a 1-1 ratio of unit to count of units that can be transported. However, on both, you can see what types the transport can take onboard by looking at the civil data page on the unit's help hover info.