Posi is nothing but GM's brand name from the 60's for limited slip. They used to call it Posi-traction. Other people called it trac-lok, power-trac, traction-loc, sure-grip, etc. They are all limited slips. A spool is not any type of posi because it isnt a differential at all (since it doesnt allow different speeds to each wheel- which is what the word differential or limited-slip differential implies).