Per Policy Aggregate is the normal limit and is an absolute limit. This is the General Aggregate Limit and considered the maximum payout for claims.
Per Project Aggregate applies the limit to each project on which you are working. This provides more coverage because the General Aggregate limit applies to each project. (Sometimes there is a maximum, sometimes there isn’t.)
Per Project Aggregate applies the General Aggregate limit to other property owned or rented and doesn’t include construction sites on which you’re working.