Wound strings use a wrap wire around a hexagonally-shaped (Hex) or a round core wire. The vast majority of modern strings (including most of ours) utilize the hex core. Hex is generally preferred because it provides greater stability, tends hold a tuning better with the small “cost” of a slightly stiffer feel than the round core.
Wound strings use a wrap wire around a round-shaped or (Round) core wire. The Round Core string option is for the round core tone purist and any player who wants the round core tone and feel. (Round core strings have a "boomier" bottom-end tone and a more flexible feel.)