Difference between a Equipment component (BOM) and a Sub-Equipament

Hello,

I still don't get the difference between a Equipment component (BOM) and a Sub-Equipament, when should I consider an item like a Sub-equipment or a component of an equiment, for me they have the same function. The only thing that I can imagene is the fact that a component (BOM) I can have in stock whereas a subcomponent I don't have a stock for it, if it brokes I will have to make a purchase requisition for its material equivalent of this equiment.

Can some one help?

Thanks a lot.