Manual Clarification Regarding Sega Gamepad

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In the Nedseq manual, under the section about Sega Button mapping, it states:

Gamepads with additional buttons (like shoulder buttons) are available, but only the
mentioned buttons are compatible with Sega systems.

Does this mean shoulder buttons can't be used with the Nerseq, or that only the listed functions can be mapped?  Or does it mean a third thing?

Thanks for your time.