The device/accessory relationship is very clearly defined in the MDD, but not at all consistently in FDA rules and guidances. It's easy enough to find statements in the regulations such as quoted above, for single regulatory issues. The problem is that there is no consistent general rule or guidance, even though such statements often are worded as if the writer assumes there is such a general rule or guidance.
Some regulations specifically mention accessories, and others don't. In my experience, this results in examiners sometimes taking the regulations literally, i.e. if accessories aren't mentioned, they're not included and must be separate devices.
FDA has major legacy issues with device definition and its classification system. There are several possible ways to classification-analyze common device types, leading to different clearance requirements. The device vs. accessory question is one aspect of this problem.
An example: X-ray fluoroscopic imaging equipment frequently is used during surgical intervention such as cardiac catheterizations to provide image-guidance in real time. Such equipment necessarily must be close to and above the invasive site, therefore it has been common clinical practice for forty years or more to apply a sterile disposable barrier-cover made of polyethylene film or another suitable sterilizable material over the upper imaging equipment components. As of 1978, FDA considered such sterile disposable covers to be accessories. As of 1982, FDA changed its approach and considered them to be devices under KKX. That however presented a definitional problem because they weren't "patient coverings", so Product Code MMP was created. But that code didn't quite align with the underlying regulation, so it's only been applied a few times since it was created, even though hundreds of such device-designs are marketed. More recently FDA has evolved back to a stance that a product that doesn't meet the specific regulation definition cannot be KKX or MMP, so it must be an accessory...except there are no guidances for sterile barrier covers as accessories.
KKX and MMP are Class II, 510(k)-required Product Codes. They have rigorous requirements...KKX is the Product Code for surgical drapes. Do non-patient-contact plastic film barrier covers need to be as highly qualified as surgical drapes? But, maybe the product is an accessory to the imaging system. Most such imaging systems are Class II, but their clearance process usually pretty much ignores the many minor accessories. Does that go too far in the opposite direction, with little or no oversight of the cover products? Also, if the accessory comes from a separate marketer than the imaging system, how is it to be regulated, as the whole concept of accessories is that they're included in the primary device clearance...? And, depending on the application of the cover, maybe instead it's an accessory to the surgical table. Surgical tables are Class I Exempt. But since there's imaging happening, maybe the table is a radiology-type, in which case it's Class II...but if it's being used for surgery, which is it? And where does the regulation go if the product will be used outside of the sterile field...since FDA doesn't even define "sterile field", even though the term is used or implied in many rules and guidances? Is it sensible to apply the same rigorous, expensive KKX requirements that would apply to an invasive-site-proximal apertured surgical drape, to a cleanliness-only footswitch control for an imaging system, sitting on the nonsterile floor?
It's a bit of an inconsistent mess, in need of a new system.