I don't have HG'22 revision, but this post by AnotherDilbert caught my attention:
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.
And it's completely.... wrong. Maybe if we were still using galleys and short-swords, I'd say that was more or less correct. But for past few centuries naval vessels (not pirates, but actual military vessels) have had either armed crew or marines as their onboard forces. A ship may have ratings trained with small arms who perform certain functions, such as acting as a guard when a ship is docked to ensure only authorized personnel come aboard. But that rating is simply pulling additional duty and isn't a "ships troop" as a singular task. Just like most ships have someone designated as a Master at Arms (or non-USN equivalent), whose job may either be, or include, the role as chief of police / security onboard a ship. The larger the ship the more space that is available for designated and specific roles.
There would be exceptions - such as a planetary navy that had regular Army troops onboard as Marine forces. That's kind of splitting hairs a bit, though I think that they'd essentially be "marines" while onboard and be treated the same from a rules perspective.
I realize it's "in the rules", but if the rules said you could repair a fusion reactor with duct tape and band aids, or that you could shoot a laser and bend the beam at a 90 degree angle 10m ahead of you to hit a target around the corner, I'd still call it wrong. I get it's a game and all, and we already make exceptions for teleporting humans in battle dress (i.e. Zho Marines).