Kassler is the traditional name in Germany. Elsewhere, butchers sell the same piece — or a close cousin — under a different name.
Kassler Kotelett is the bone-in chop variant — same cut as a pork chop, but brine-cured and cold-smoked rather than sold fresh.
Same anatomical cut — Kassler is the brine-cured, cold-smoked form of the fresh pork loin. Anglo-American "Canadian bacon" is similar in concept (cured loin) but is fully cooked rather than smoke-cured.