What are these? And how am I affected by these as a developer?
Related:
What are the different calling conventions in C/C++ and what do each mean?
A calling convention describes how something may call another function. This requires parameters and state to be passed to the other function, so that it can execute and return control correctly. The way in which this is done has to be standardized and specified, so that the compiler knows how to order parameters for consumption by the remote function that's being called. There are several standard calling conventions, but the most common are fastcall, stdcall, and cdecl.
Usually, the term custom calling convention is a bit of a misnomer and refers to one of two things:
A non-standard calling convention or one that isn't in widespread use (e.g. if you're building an architecture from scratch).
A special optimization that a compiler/linker can perform that uses a one-shot calling convention for the purpose of improving performance.
In the latter case, this causes some values that would otherwise be pushed onto the stack to be stored in registers instead. The compiler will try to make this decision based on how the parameters are being used inside the code. For example, if the parameter is going to be used as a maximum value for a loop index, such that the index is compared against the max on each iteration to see if things should continue, that would be a good case for moving it into a register.
If the optimization is carried out, this typically reduces code size and improves performance.
And how am I affected by these as a developer?
From your standpoint as a developer, you probably don't care; this is an optimization that will happen automatically.