First off I hate to be bearer of bad news but unless you’re a diamond you probably won’t get to choose what unit you go to. But to answer your question that’s not really how specialisations work. 43 is different because they have a specific job role so they have no need for fire support group or mortar troop or anything like that. At the other units each has a mix of different specialisations in order to meet their requirements. Bare in mind that everything is changing due to future commando force, but in the old way of doing things at a unit let’s say 45 you would have 3-4 coys or squadrons, each made up of 3-4 troops of GD marines and the troop cpls/stripeys could be a range of different SQs. These are fighting troops. Then on top of that you will have fire support group (FSG) made up of heavy weapons, PW gunners, anti tanks etc. Then there’s also mortar troop, recce troop will consist of your MLs and snipers. Medics will be attached to most fighting troops etc. Each camp has a motor transport dept where the drivers live and stores where the store men live etc. Do you see where I’m coming from?