subset (n.)

"collection of matching things," mid-15c., sette, sete, earlier "religious sect" (late 14c.), in part from Middle English set, past participle of setten (see set (v.)) and in part from Old French sette, sete "sequence," a variant of secte "religious community," from Medieval Latin secta "retinue," from Latin secta "a following" (see sect).

Skeat first proposed that set (n.), in the sense of "a number of things or persons belonging together" ultimately was a corruption of the source of sect, influenced by set (v.) in subsequent developments as if meaning "a number set together." Thus this noun set was in Middle English earliest in the sense of "religious sect," which also likely developed some modern meanings, such as "group of people" (mid-15c.), especially "persons customarily or officially associated" (1680s); "group of persons with shared status, habits, or affinities" (1777).

The meaning "a number of things having a resemblance or natural affinity; complete collection of pieces to be used together" is by 1560s. Hence, "collection of volumes by one author" (1590s), "complete apparatus for some purpose" (1891, of telephones, radio, etc.).

Meaning "group of pieces musicians perform at a club during 45 minutes" (more or less) is from c. 1925, though it is found in a similar sense from 1580s. Set-piece is from 1846 as "grouping of people in a work of visual art;" from 1932 in reference to literary works.

The word sett is a variant, preserved in old law and "now prevalent in many technical senses" [OED].