Below species epithet you have subspecies (ssp. or more properly subsp.), which is a published taxonomic name, and forma (f.) which is sometimes published (and then usually in Latin) or just a description of some type of natural variant, e.g. f. alba for the white flowered form of something that's normally pink; or just white form. A cultivar name is that of an actual clone, and is written in single quotes: Erythronium 'Citronella'. A recognized but non-clonal group grown from seed is called a strain or group, and strictly speaking is distinguished by a capital letter alone: Eccremocarpus scaber Tresco Strain (or Group). If gardeners habitually use a plant name that is not published or otherwise recognized, you can put it in double quotes: Erythronium multiscapideum "Cliftonii". The last example shows a name that was thought to identify a subspecies but is not accepted by taxonomists; still, growers use it to denote plants from a certain population with unusually large flowers. When one has to identify something whose taxonomic status is confused, one can write, e.g., Narcissus received as loiseleurii, and blame the person who sent it to you. You don't need to know all this except for publishing, and then you hope for a good editor.