On the site you will see our polyester series broken down by ester or acid endcap. This is based on the initiator used to start the materials with. Ester endcap materials are started with a decanol (10-carbon alcohol) so there is a decane group remaining on the end of the acid side of the chain. The acid endcap materials are started with loose lactic acid or just water so the acid side is left free as an acid.
There is a subtle impact from this endcap effect however which is described in part in a recent thesis available in full-text here.