There isn't one. The regulatory aspects of what parts are "legal" to install in an airplane are comprised of a scattered mess of Advisory Circulars, FAA Orders, and vague regulatory wording (like "Approved Data" and "Data Acceptable to the Administrator."). With respect to TSOd equipment (not equipment that "meets" TSO, but that actually has a TSO), the FAA has said this part is acceptable to be installed in airplanes. That gives an A&P a lot of comfort and takes the burden (and liability) of that decision off his/her shoulders. Now that A&P just has to decide that the TSOd part can be installed in THIS airplane, which is generally an easier task. With STCd parts, the FAA has said this particular part can be installed in this particular airplane, which makes things REALLY easy for an A&P. You can see why A&Ps like TSOd stuff, but really, really prefer STCd stuff (with ultimate preference being factory parts). As far as I can tell, there is no rule that says an A&P can or cannot install non-TSOd, PMAd, or STCd parts on an certified airplane; the A&P has the legal authority to make that ultimate airworthiness decision and install the part. However, if the FAA decides the A&P was wrong, trouble is likely. If you're a working A&P counting on that job as your livelihood, what would you do?