CDS Cheapest to Deliver

A little confused as to why we choose the bond we choose. is it just based on seniority. if there are 2 bonds with the same seniority, then which do we choose?

The one that is trading at the lowest % of par value; i.e. the cheapest one, given that the bonds are ranked equivalently (or higher) in the priority of claims to the reference obligation (pari passu).