In your example, you just assigned at AX the value 23. The link to pins come only later on, when you call functions that expect a pin number like
In this case, you could also write 23 instead of AX, but it might become a mess if you decide to change the pin to 22. In the first case you just change the value of AX in one place and everything else falls in place. If you'd have sprinkled 23s across your code, you'd have to change them all and at the same time make sure you don't change any 23 that aren't used as references to the pin.
Another common point of confusion is that in the Arduino library, the anaolgue pins and digital pins are both numbered from 0. So
reads date from another pin as