There seem to be several claims. Mentha x piperita (peppermint), M. spicata (spearmint) , M. suaveolens (apple or woolly mint), and M. arvensis (wild or field mint), have all been called sweet mint. Best guess, the name commercially might usually refer to some form of spearmint. M. arvensis grows in the wild, but not as common to find commercially. One vendor sells sweet mint but separates it from basic peppermint and spearmint - unfortunately, no botanical name listed.
There seem to be several claims. Mentha x piperita (peppermint), M. spicata (spearmint) , M. suaveolens (apple or woolly mint), and M. arvensis (wild or field mint), have all been called sweet mint. Best guess, the name commercially might usually refer to some form of spearmint. M. arvensis grows in the wild, but not as common to find commercially. One vendor sells sweet mint but separates it from basic peppermint and spearmint - unfortunately, no botanical name listed.