EPDM pond liner is made from M-class synthetic rubber. The M refers to its classification in ASTM standard D-1418; the M class includes rubbers having a saturated chain of the polymethylene type.
In other words, it keeps water in, won’t poison your pond or yard with toxins, and doesn’t damage easily.
Brian only understood the last sentence, which makes sense because he isn’t a liner expert. As long as it does a great job and doesn’t hurt anything that’s all he needs.