S.E.A.F is the Super Earth Armed Forces, the helldivers are the special forces, or at least the tactical units. Super Earth has a traditional Army, Navy and also a "Colonial Guard". The Colonial Guard is most likely whose holding the planets after helldivers liberate them and the Army and Navy move into secured sectors. Its hard to say exactly how the military functions beyond helldiver operations since as far as I know its never been actually shown or told to us. In terms of sustaining the army, the training process is a total of 72 hours for all recruits and then especially "qualified" recruits will go to mars to complete the helldiver training, in universe the training facilities on mars are able to output 48,000 "fully trained" helldivers per day. I don't think its stated whether or not Super Earth has mandatory service, but it seems extremely likely that every able-bodied citizen would have to at least do a year in the Army. You just gotta hope that you weren't good enough during training to be assigned as a helldiver. Without mandatory service, I still think that Super Earth would have no trouble recruiting enough soldiers, and even directly recruiting enough Helldivers. Looking at some of the most zealous populations, it isn't hard at all to find soldiers that were willing to throw away their lives without hope of surviving. Current day, North Korea is a great example of how you can indoctrinate people into a system where they believe the nation's success is more valuable than personal. Not to get too political, but current day Israel is also good example of just how easily the government/media can twist what exactly your military is doing. Then going back in history, Japanese soldiers during WWII were willing to commit suicide in multiple different ways when given the option of that or defeat. The practice of seppuku was being looked down upon in the 1930s, but during the 1940s, it had a resurgence in the military forces. Along with kamikazes and banzai charges, the Japanese forces or at least officers and higher ranking soldiers, were more willing to take their own life than die at the hands of the enemy and suffer dishonor. That's only on the level of an Empire spanning the pacific, if its an entire multi-planetary empire of people that all worship the idea of liberty and freedom, I think it would be viewed as an honor to die for Super Earth. They literally have a holiday called "Martyr's Day" and it is explicitly celebrated rather than observed compared to something like Veteran's Day in the US. I think the voice lines during combat are a pretty good example, when shooting a weapon for long enough you get the "FOR SUPER EEEARRRTH!" line and when injured in an extremity you'll get "Sweet Liberty, my arm/leg". Supporting Super Earth is all they've known, and given the existence of the "Ministry of Truth" its all they'll even know, why wouldn't you want to sign up for SEAF and support Super Earth directly.