The so-called military service model is literally joining the military. One becomes a member of the military, for which the army provides various weapons and training, amongst other things. Those who participate must strictly adhere to the regulations and rules.
It can be said that after joining the military, there is no more freedom, everything must be done in accordance with commands.
The advantages are that you can obtain assistance from the military and it is also safer. There are more chances to participate in battles, making it easier to earn military merits. It is arguably the easiest path to take.
However, the downside is that everything must be strictly done according to military requirements. Many nobles cannot handle this method, which is why a second method emerged later.
This second method is quite similar to the situation Du You encountered in the Derived World during the Nordland Domination.