HG'22, p92:
MARINES AND SHIP’S TROOPS
The terms marines and ship’s troops are sometimes used interchangeably. While the two share some of the same duties, there are significant differences in scope.
Ship’s troops are exactly that: troops on a ship. The primary role of the ship’s troop is to defend their ship. When the ship is docked to another, their duties may extend to the attached ship, whether they are sent inside as a security detachment, part of an inspection or a boarding party.
Marines are often used for the same duties but are also on hand for standard marine functions such as orbital drops and raiding. Marines also serve in lift-infantry units but this requires a special type of ship, usually a capital ship that is a member of an AssaultRon (assault squadron). Lift infantry usually include a vehicular component that requires a capital ship to transport it.