Page 2 of the Core rules dictates how armies are Setup. It specifically calls out Battleplans handling the setup specifics. These are more specific than core rules, and trump them. Units would not be able to garrison within the RSE if it is within the specified parameters of the specific battleplan. Which is generally "Cannot deploy within x" of an enemy territory". Therefor, you cannot deploy a unit in the RSE, even if it is within your territory, if said RSE is within the specified range of the enemy territoy.