UNSC marine corps - Launching land based operations from naval ships. The marines cooperate directly with the navy due to obvious reasons. The whole marine corps is an extension of the navy.
UNSC army - A part of the UNSC ground forces. The army is defending and offending enemies on known soil. It has happened before that the marines have suggested army presence when lacking manpower. Therefore the army has two administrations, the administration of defense and the administration of offense.
UNSC self defense force - A part of the UNSC ground forces. A fairly new addition to the defense family of the UNSC. The SDF suggestion was accepted in 2567, colonial militia was to be replaced by a professional stand alone branch.
The SDF is capable of policing duties aswell as defending high value targets and buildings. Basically the SDF is a mixture between the police force and the army.
A mere excuse to justify martial laws, with great success.
Description is made up to match the RECON comic. Questions are welcome, but you don't have to talk about stuff that we all already know.
Alright, pick your favourites people, I know my vote is for the green guys!