This was somewhat true until about a month ago, when the entire system was Tangoized. Everything in the GDS is now mixed, and you really have to know what codes Tango is (NGPEA dom/usa/asia... TLK europe/sam)
thanks for the update there, I was referring to transborder/intra-canada fares. It seems the TATL fares earn 100% on UA, so they're OK... (but they are more restricted in general).