Prior to the 'classical' era (beginning ca. 1750) compositions were much more flexible in terms of performing forces. Music of this time period is often referred to as "Consort" music.
Possibly related to the Italian word 'concerto', the musical consort was
"A small instrumental ensemble for playing music composed before about 1700. The meaning is frequently extended to cover ensembles of voices with or without instrumental accompaniment, and the word is also applied to the music itself." - Grove music online "Consort"
Because of it's relatively flexible nature, we have chosen to shelve all music of this period for multiple performers in one class number, organized by composer. Thus you will find Gibbons' "Consort anthems" alongside his music for 3 viols in SCO 785.1 G441
The one exception to this is music for individual instruments. Music for solo viol will be in SCO 787.6 with music for stringed instruments (Music: 78; strings: 7; viols: 6). Note that (in contrast to modern instruments) music for multiple viols will be placed with Pre-18th Cent. ensemble music in SCO 785.1