A "flag bid," again, is a call that "flags" a specific suit and sends another message. A classic example might be a 1NT opening and a 3S response, if 3S shows 5-5 in the majors and GF. Opener could decline a major (3NT), or pick a major (4H or 4S). However, a more sophisticated approach would allow Opener to "flag" a major (4C for hearts, 4D for spades) as a super-acceptance.
But, flags are not necessarily only for slams. Consider other situations.
How about a flag as a lead-director?
The opponents open 1C to your right and respond 1H to your left, partner intervening with a 1NT call, Sandwich, showing spades and diamonds. Simple is to bid 2S or 2D, picking your preference. However, consider this. What if you want to support one suit but would prefer the other suit for lead? Maybe xxxx-Kx? One could easily imagine that you "transfer" into a suit to show one thing and bid it directly for another. 2C might say, for instance, that you prefer diamonds but want a spade lead, and 2D says that you prefer diamonds and want a diamond lead. 2H shows spade preference for play but diamond preference for lead; 2S preference for spades on both accounts.
How about to distinguish preemptive or preference from game-invitational?
Partner overcalls 1NT by bidding 2H, majors. You could pass or correct. However, maybe you want to jam the auction (3H or 3S). Maybe you actually want to invite game. 2NT might work as the invite call, but how about flags? Or, what about if 2NT shows the majors and a strong hand? You could simply guess the level, or use flags to invite.
Now, I'm not going to suggest a whole new system, where 1C and 1D openings are flags for the majors, but there are a lot of uses for this approach beyond the simple slam try.