Is a pointer parameter an input, output, or both?
You are passing in a memory location that can be read or written too.
That’s it.
Address zero exists in the CPU, but that's not the null pointer, that's an embarrassment if you happen to need to talk about address zero in a language where that has the same spelling as a null pointer because you can't say what you meant.
It's OK that you didn't know this if you mostly write C++ and somewhat OK that you didn't know this even if you mostly write C but stick to pre-defined stuff like that NULL constant, if you write important tools in or for C this was a pretty important gap in your understanding.
In C23 the committee gave C the C++ nullptr constant, and the associated nullptr_t type, and basically rewrote history to make this entire mess, in reality the fault of C++ now "because it's for compatibility with C". This is a pretty routine outcome, you can see that WG14 members who are sick of this tend to just walk away from the committee because fighting it is largely futile and they could just retire and write in C89 or even K&R C without thinking about Bjarne at all.
I worked on a massive codebase where we used Microsoft SAL to annotate all parameters to specify intent. The compiler could throw errors based on these annotations to indicate misuse.
This seems like an extension of that.
A language like C# has true directional parameters. C only truly has “input”