How does a UN force operate?

I never really create my figures with any particular background in
mind, and thus came up with the idea of having a UN force for an
upcoming SG2 game-- I just throw all my figures together to make an
army composed of different nationalities.

My question, then, is "How does a UN force operate?"  Do local forces
conform to any UN organization, or does the UN force organization
revolve around the local forces supplied?  To what extent is the UN
able to dictate what forces are necessary (such as air war vs. ground
war, or requiring more tanks, etc...?), or do they pretty much take
what is supplied and work with that?  To what extent will given orders