What parts of dash are not part of the POSIX standard?

I know that dash adheres quite closely to the POSIX standard, but I know that it's not 100.00% strict POSIX without any extras. The shell that comes closest/adheres exactly to the POSIX standard is as far as I know mrsh or yash with the POSIXly-correct flag set. Now I want to know exactly which parts/functionalities of dash are not specified in the POSIX standard (without reading the entirety of [POSIX.1-2017](https://pubs.opengroup.org/onlinepubs/9699919799/utilities/contents.html) and the source code of dash). I already tried extensive googling (as far as I'm knowledgable enough for this topic) but who would have thought, it's exclusively results on what the differences between bash and dash are and how dash is very POSIX compliant and so on.