People often order poly around the entire perimeter of a unit when the hair line is not exposed. The reason for a lace front is to give a natural hair line. But if you're not exposing the hair line (combing forward) it's not a concern. For newbies, poly is a 3/4" wide plastic strip that provides a flat smooth surface to attach glue or tape. There's two advantages to poly. First, you have more surface area to attach to than lace with provides a stronger bond. Two, cleanup is easier since you don't have to clean lace--just swab the poly with Lace Release or rubbing alcohol to clean the poly.
Regards,
John