Bodygroups for custom models.

I’ve been using an example of a QC file in order to create the QC I need to compile and work on my current model project, but I’m new to the whole thing and have gotten a little confused with the bodygroups command. Because the example is using the source citizen model it has lots of bodygroups available, but I’m a little confused as to what bodygroups do, if I need any for a custom model, and how I create them.

From what I can kind of deduce from the file, it seems that it might have something to do with if a model has multiple parts that can be interchanged, like the citizen model, which does indeed have multiple parts that offer variety, this also seems to be the case with the skingroups, though if I wanted my model to have a single texture and model that it was going to use, I’d imagine I would still need a single bodygroup, which goes back to the question of how I would make one of these for the model I am aiming to convert into a player model.