The operand constraints have another function: they enable the compiler to decide which kind of hardware register a pseudo register is best allocated to. The compiler examines the constraints that apply to the insns that use the pseudo register, looking for the machine-dependent letters such as d and a that specify classes of registers. The pseudo register is put in whichever class gets the most “votes”. The constraint letters g and r also vote: they vote in favor of a general register. The machine description says which registers are considered general.
Of course, on some machines all registers are equivalent, and no register classes are defined. Then none of this complexity is relevant.