It is an odd curiosity of mine. It says
Designed for N64 Expansion Pak
on the box, but I can't find the answer on Google. Does anybody know what the game uses the Expansion Pak for?
1 2 Neither of these lists show that the Expansion Pak does anything for Rainbow Six. This review claims that the Expansion Pak boosts the resolution ("A CRT TV still offers the best experience, especially when using the N64 Expansion Pak to boost the resolution."), but I tested it in emulation and this is not the case. The only other semi-professional source that says anything about Rainbow Six and the Expansion Pak is a page on the German Wikipedia that says it does something, but not exactly what it does.
So the review is flat out wrong, the German Wikipedia is incredibly vague, and the other 2 sources and my personal tests in emulation show that the Expansion Pak does nothing. Those were literally the only semi-credible sources I could find about Rainbow Six and it's (lack of?) Expansion Pak support, and that fact points to "the Expansion Pak probably does nothing for Rainbow Six". I'd say it's safe to assume that the Expansion Pak is useless for Rainbow Six. Even if it did do something that everyone completely missed, whatever it does probably doesn't matter since nobody was able to find it.