If you add a second body to your part, the first operation can actually be a negative body (i.e. Hole, Pocket, etc).
If you perform an Add of this body, it forces the polarity to be positive, and treats the final shape of the body as a lump of material to be added to your first Body.
If you perform a Subtract of this body, it forces the polarity to be negitive, and treats the final shape of the body as a lump of material to be removed from your first Body.
If you perform a Assemble of this body, it uses whatever polarity that the body contains. If it's is negative, it subtracts. If it's positive, it adds.
Normally, this isn't something you would use day in and day out. It is extremely powerful, however, when using any of the Design Automation tools such as PowerCopies, User Defined Features, Templates, Knowledgeware, etc.