Just to add to Jazz's comment, and possibly to head off the next question, apart from position (limit switches on the extremes of travel, home switches at some arbitrary position within the extremes of travel - possible a limit switch) - the distinction is made within the Mach3 settings - you'll bind an input (Pin 11/12/13 - assume X/Y/Z, according to the silk-screen printing on the board) to a function (limit/home) within the Mach3 inputs, then Mach3 takes over from there.