Below are four important definitions from FDA guidance document entitled: "Medical Device Accessories – Describing Accessories and Classification Pathways".
Accessory: A finished device that is intended to support, supplement, and/or augment the performance of one or more parent devices. Component (21 CFR 820.3(c)): “[A]ny raw material, substance, piece, part, software, firmware, labeling, or assembly which is intended to be included as part of the finished, packaged, and labeled device.”
Finished Device (21 CFR 820.3(l)): “[A]ny device or accessory to any device that is suitable for use or capable of functioning, whether or not it is packaged, labeled, or sterilized.”
Parent Device: A finished device whose performance is supported, supplemented, and/or augmented by one or more accessories.
I hope this can helpful to you.
Shimon