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Description
A squad of Terran Protectorate Colonial Marine Corps troopers securing a research facility.

The Colonial Marines are named such because one of the requirements of being a member planet of the Protectorate is to raise a regiment of marines separate from your planet's own militia to serve offworld with the Marine Corps. Thus each regiment is known by its home colony. In most cases, planets raise multiple regiments, especially if the population is large.
Regiments tend to be specialized in one particular area excelled at by the colonists on their home world. So a regiment raised from the icy planet Eiswelt might be arctic combat specialists, while a regiment from the planet York, known for its massive shipyards, may specialize in boarding operations.
Colonial regiments are known by their planet name,the regimental number, and their specialty. So the first regiment ever raised by Viridia III, focused on mechanized infantry, would be the Viridian First Mechanized Infantry, and the fifth regiment raised by the Alexandria colony, focused on reconnaisance, would be the Alexandria Fifth Recon.
Regiments made entirely of members of the same planet have been found to work better together in tense situations, are less likely to have communications problems due to language or cultural misunderstandings, and have a strong espirit de corps.

The Terran Protectorate does not have a standing army for the entire protectorate. The marines serve that purpose. Armies and militias are specific to each colony and limited to their home star system.

Description
A squad of Terran Protectorate Colonial Marine Corps troopers securing a research facility.

The Colonial Marines are named such because one of the requirements of being a member planet of the Protectorate is to raise a regiment of marines separate from your planet's own militia to serve offworld with the Marine Corps. Thus each regiment is known by its home colony. In most cases, planets raise multiple regiments, especially if the population is large.
Regiments tend to be specialized in one particular area excelled at by the colonists on their home world. So a regiment raised from the icy planet Eiswelt might be arctic combat specialists, while a regiment from the planet York, known for its massive shipyards, may specialize in boarding operations.
Colonial regiments are known by their planet name,the regimental number, and their specialty. So the first regiment ever raised by Viridia III, focused on mechanized infantry, would be the Viridian First Mechanized Infantry, and the fifth regiment raised by the Alexandria colony, focused on reconnaisance, would be the Alexandria Fifth Recon.
Regiments made entirely of members of the same planet have been found to work better together in tense situations, are less likely to have communications problems due to language or cultural misunderstandings, and have a strong espirit de corps.

The Terran Protectorate does not have a standing army for the entire protectorate. The marines serve that purpose. Armies and militias are specific to each colony and limited to their home star system.