An accessory cloud is a cloud accompanying or adjacent cloud mass to another cloud, generally smaller than the latter, and separated from its principal part, usually a major cloud genus (or type) or sometimes partially merged with it. A specific cloud may be accompanied by one or several accessory clouds. Accessory clouds are:

arcus (arc) - arch, shelf, roll or wall cloud
incus (inc) - anvil
mamma (mam) - pouches hanging from underneath main clouds
pannus (pan) - ragged shreds of cloud
pileus (pil) - cap cloud
praecipitatio (pra) - fall, precipitation reaching the surface, usually in the distance
tuba (tub) - funnel clouds
velum (vel) - thin layer of veil
virga (vir) - fallstreaks, trailing to the ground
and last but not least the banner cloud

Related features:
Cap cloud
Cloud classification
Cloud types (genera)
Cloud variety
Clouds - sentry of the sky