The difference revolves around the nature of the plant. Things like Amazon Swords and Cryptocoryne are "Rosette" plants which have a root, a crown and leaves growing from the crown. These are typically sold as individual plants, i.e. one crown, with roots below and leaves above.
Things like Cabomba, Hornwort, Bacopa etc., are "Stem" plants. These may have a root stock, but from this, long stems grow with leaves appearing at lengths along the stem. typically these are sold as a number of cuttings, i.e. pieces of stem which will often root if treated correctly.
There are other types, Anubius sprecies for example which grow from a rhizome, these, again, are typically sold as "a plant" meaning a piece of rhizome with a few leaves growing from it.