I understand that memory has to be reserved before being committed. And when it's reserved, no other process can use it. However reserved memory does not count against available RAM. But shouldn't it? Because if no one else can use it, then what good is it being "available"?
Or is there some bigger difference?
In the context of Win32, "reserved" means that the address space is allocated within the process that requested it. This may be used, for example, to reserve space for a large buffer that's not all needed right away, but when it's needed it would need to be contiguous.
Reserving memory does not interact with other processes at all, since each process has its own private address space. So the statement that "when it's reserved, no other process can use it" is meaningless, since processes can't normally allocate memory in the address space of another process anyway.
When the reserved pages are requested to be committed (backing store allocated for them), that operation can potentially fail due to lack of physical memory (or pagefile).