Stuped question I have a 6 button joystick just want to know what about button mod upper joystick and how to use
Is this what you have? Those are not usually called Joystick though. They usually go as "controller" or "gamepad". I think the "mod" button is only for booting some games that have compatibility problems with the 6 button pad. You would hold it during game startup to prevent issues.
As MaxWar said the controller model MK 1653 is the original Sega 6 button model. The button you call "mod" is actually labeled "mode". This changes the controller operation from that of being 6 button with A,B,C,X,Y,Z to that of a 3 button with A,B,C buttons. You hold the "mode" button with turning the console on to change to the 3 button mode since a few games had issues with 6 button controllers. There are a few games that actually used the mode button ingame but they are very few and far between.