ModX in event.state in tkinter?

Question

I've been figuring out how to parse tkinter events via event.state to reduce the number of times that I have to call root.bind() (e.g., I can avoid binding both "<ButtonPress-1>" and "<Shift-ButtonPress-1>" by finding if shift was pressed via event.state). Of course, I've relied heavily on the tkinter source code (specifically the definition for __repr__, starting on line 234) to convert the integer of event.state to something I can understand:

def getStatefromInt(state_int):
    # from https://github.com/python/cpython/blob/3.8/Lib/tkinter/__init__.py
    if isinstance(state_int, int):
        state = state_int
        mods = ('Shift', 'Lock', 'Control',
                'Mod1', 'Mod2', 'Mod3', 'Mod4', 'Mod5',
                'Button1', 'Button2', 'Button3', 'Button4', 'Button5')
        s = []
        for i, n in enumerate(mods):
            if state & (1 << i):
                s.append(n)
        state = state & ~((1<< len(mods)) - 1)
        if state or not s:
            s.append(hex(state))
        return s

One of the things that keeps coming up out of state when events occur is Mod1. What do Mod1 and the other ModX states represent? I thought the number might correspond to the type of button press, but all types of mouse clicks cause only Mod1. I have not been able to find information on what this means online, and I'm having a hard time seeing from the source code what it might mean.