While it’s a bit of an apples to oranges comparison across different programs, I’ve found that generally most *A airlines board their highest internal tier first, and the next boarding zone is where they place *G passengers from other airlines. Certainly the case for AC, UA, LH, NH, so not surprising for BR too. (Well, I suppose UA technically boards their top 2 before *G but the Global Services tier is but unique)
SK was one of the outliers in this regard giving zone 1 to *G. With the automated BP scanners at many European airports it actually made a difference too unlike the typical AC flight.