What do the red exclamation marks on the Boosts screen mean? (AB2)

The red exclamation marks on the Boosts screen simply denote any new boosts you've picked up since the last time you looked at that screen. E.g., let's say that you look at the Boosts screen now, then play a match where you earn a "Ban XZJQ" boost. The next time you visit the Boosts screen, Ban XZJQ will have a red exclamation mark on it, indicating that you've picked up at least one new Ban XZJQ boost since your previous viewing of that screen. If you then close and immediately reopen the Boosts screen, the red exclamation mark will have disappeared because you haven't picked up any new Ban XZJQ boosts in the few seconds that the screen was closed before you viewed it again.