The short version is that the Royal Marines are a commando unit designed to provide raiding, scouting, and specialist expertise to Naval and Army operations. They do not have a combined arms element. There are elements of the Royal Army trained to provide artillery support for RM operations and, likewise, elements of the Royal Navy's Fleet Air Arm that are trained to support the Royal Marines. But, other than a small logistics element, the Royal Marines are an all infantry commando force. They are integrated command wise into the Royal Army when deployed ashore and revert to the Navy when afloat.
The USMC have developed into a distinct combined arms branch. Though they have recently scaled back on their integral armor, they have maintained their own fixed and rotary wing pilots, armor, artillery, and support elements. They have commando elements in the USMC, but that isn't the totality of Marine Corps role.
Traveller has generally used the USMC model, including their distinctive ranks and generally suggesting that the Marines are trained and equipped for extensive long term planet side operations, with their units have integral artillery and armor elements, with some sources stating that they have their own starships (with Navy crews)