3rd member

Ok guys I have a question. I am running a 9" Ford rear end with a 3.89 posi gear ratio . I was looking for a lower ratio gear set say 3.70 I found one and asked the company if this was a posi rear end and he told me they, as in their company, dont make posi they only make solid spools. So my question is what is a solid spool compared to a posi?
Thanks