Within STF there are a number of other Branches, some that are extrapolations from Canon and non Canon sources and some that are explained in a fair amount of Canon detail.
Marines are an interesting group that are only used on one ship, the U.S.S Endeavour in Fleet 5. There were no direct references to Marine units in any of the Star Trek series other than Enterprise in which they were used quite effectively as a Standard shipboard force. They have received passing nods in a number of episodes of the Next Generation and Deep Space Nine and would undoubtedly have been used as ground combat troops during the Dominion War. The specific rules governing Marines on the Endeavour can be obtained from the CO. They are a little too long winded to post directly in this course, but generally speaking Marine spots are only given to more experienced players because of the difficulty found in accurately portraying such a difficult field.
Intelligence is my personal favourite and my active COS characters both have Intelligence backgrounds, though it's also a sort of grey area, if you'll forgive the pun. The only active Intelligence position in STF is the swing position of Chief Intelligence Officer, which, like all swing positions is dependant upon the CO in question and is notoriously difficult to play. There have been a number of episode in Deep Space Nine and several that I can think of in The Next Generation dealing with Intelligence. Every Trek fan is familiar with the Section 31 storyline that ran through Deep Space Nine, though it is worth noting that not all Intelligence branches are quite so evil. CIO's generally have very little to do unless the sim in question has an Intelligence undertone, though it can be very rewarding and a lot of fun. If you choose an Intelligence related character it's a good idea to do a lot of research into the field in the 21st and 24th Centuries to help flesh them out. Remember though that having your character hail from Section 31 is highly unlikely as that is an organisation from which you do not leave lightly. If there are any specific questions about Intelligence theory, practice or Character development don't hesitate to contact me at andrewrobinson257@msn.com.
The position of Chief Tactical Officer is a swing position in STF, much like the CIO and Marine spots. The presence of a CTO on a vessel somewhat alters the position and responsibilities of the Chief of Security in the sense that the CTO becomes the officer primarily assigned to the Tactical station on a Starship Bridge. Tactical Officers are tasked with the ships combat operations and is generally only present on combat oriented vessels because of the nature of the position. In this situation Security Officers responsibilities no longer extend to ship to ship combat, as that's handled by the CTO. Obviously because of the limited scope of the characters duties it can be difficult to play a Tactical Officer and within STF the line between CTO and COS is often blurred by circumstance.