It's the Colorado flag. Pretty sure most Yeti bikes built in Durango had these, but I'm sure a Yeti-guru will confirm exactly what models and build years that exact sticker applies to .
What frame do you have slimjoe? I bought this years ago off a seller that had a whole bunch of mint nos ultimate and fro decals. I haven't seen it on any of these frames though.. So I'm a bit stumped.
My frame is a Sherpa 96 one year only for this re-badged FRO. It had that very sticker on the top of the seat tube but I had to have it re-painted after a comprehensive restoration