On January 20, 2015, FDA-CDRH published a draft guidance document, UCM429672, Medical Device Accessories: Defining Accessories and Classification Pathway for New Accessory Types. The intent is define when an article is an accessory and for accessories determine the risk and control levels.
The draft guidance offers some definitions.
Accessory: A device that is intended to support, supplement, and/or augment the performance of one or more parent devices.
Parent Device: A finished device whose performance is supported, supplemented, and/or augmented by one or more accessories.
It also includes addition information on use with a parent device, as well as a discussion of the terms support, supplement, and augment.
The docket number at www.regulations.gov is FDA-2015-D-0025. At the docket, you may read the comments posted.