You can configure three MPG's in Config>Ports & Pins >Encoder/MPG's and each would have its owne pin and port assigned to it.
So Alt A is MPG number 1 in configuration, BUT , as you click Alt A you will see via the green LED that
the axis associated with MPG#1 will define what axis it will be used for. So if X,Y,Z,A,B,C axes are actualy configured in Mach it will jog that axis.
An example:
I use two MPG's but for the lathe they are assigned to X & Z.
For the mill they are normaly used / assigned X &Y but if required I can assign one of them to Z if so desired by click AltB button.
RICH