In the past, T meant economy if AY prime code but premium economy if AY codeshare on other metal. As AY is in the process of introducing premium economy, T was retired as economy and is now always supposed to mean premium economy, but it's possible that the mapping is confusing to BA.

Finnair used to have a number of fare buckets which meant one class of travel if AY metal and a different class of travel if AY codeshare. I think that the only remaining confusing fare bucket is A which means economy if AY metal (special tickets only sold through certain travel agencies) but first class if AY codeshare on AA or BA metal.