An example for the force is - output slot for machines, crafting benches. You should not be able to put anything in there, however when the machine produces something, it should be able to force the item into the slot.
I'm not sure I understand the second question. If you are referring to, what would happen with the item, if inventory would no accept it, I could add a boolean flag to the constructor that would allow automatically destroying it in this case, that would at least make the caller think what to do.