"CivetOne" wrote in message
...

snip
The § 21.101 rule mentioned in § 21.115(a) prescribes the applicable
airworthiness rules. In our example, the prescribed rules would be found
in
part 23. There are no provisions for issuance of STCs or PMAs in part 23.

The only thing Part 23 is about is STCs. In fact, that is the title of Part
23.