How can an abi allow but not require passing them via registers? Isn't the point of a convention that everyone does it the same way? Other wise the caller could

How can an abi allow but not require passing them via registers? Isn't the point of a convention that everyone does it the same way? Other wise the caller could put it on the stack and the callee look for it in a register? Actually reading the word abi made a little neuron light up. To what extent does rust even follow that abi?