supplemental (adj.)

late 14c., from Latin supplementum "that which fills up, that with which anything is made full or whole, something added to supply a deficiency," from supplere "to fill up" (see supply (v.)).

suffix forming adjectives from nouns or other adjectives, "of, like, related to, pertaining to," Middle English -al, -el, from French or directly from Latin -alis (see -al (2)).

1660s, "added as something extra," from supplement (n.) + -ary. Suppletory in the same sense is from 1620s.