> is there a common standard for how they are using the pins?
None that I'm aware of. Considering some pinouts I've seen my guess is that different vendors simply assign them as they see fit. Also, especially ARM SoCs have widely varying capabilities, and different SoMs have all kinds of different hardware on module/off module, different bus systems they support, and so on. Also, some SoCs can change some of their pin assignments to a degree. Given all that and considering that the whole purpose is usually to cheaply develop semi-customized embedded hardware for specific applications, there might not be much demand for that either.
None that I'm aware of. Considering some pinouts I've seen my guess is that different vendors simply assign them as they see fit. Also, especially ARM SoCs have widely varying capabilities, and different SoMs have all kinds of different hardware on module/off module, different bus systems they support, and so on. Also, some SoCs can change some of their pin assignments to a degree. Given all that and considering that the whole purpose is usually to cheaply develop semi-customized embedded hardware for specific applications, there might not be much demand for that either.