War Thunder? If so, Gaijin's latest dev diary video is worth watching. The clip is the first in a planned series called Developer Answers. It runs for nearly nine minutes and explains, in part, how BR is the average efficiency of a given vehicle in all battles as opposed to the efficiency of a given vehicle against another specific type.
Various War Thunder devs explain how this relates to the WWII flight sim's matchmaking mechanics, too, and they also address several other questions pulled from community members and social media channels. You can watch the full clip after the cut.