Game Flags are indicators of some characteristic, such as "Brilliant Play" or "White Opening", and are used for classifying games and enabling fast database searches. There are 12 user flags and 6 custom flags
Flags are set in the Maintenance Window. Select the desired flag from the menu-button, and then use the appropriate "Mark" button. The Delete flag has its own buttons, but it is just a special-case flag which indicates games will be removed when the database it is next compacted. Scid vs. PC also allows quick flag tagging via the Game List context menu.
The User Flags are
The Custom Flags (1 to 6) have labels up to eight characters long, which are changeable in the Maintenance Window.
You can use a Header Search to find games with a particular flag turned on or off, or use flags as part of more complex searches.
Since all the user flags (except Delete and Tactics) have no special significance, one may use them for any purpose. For example, you could use the Kingside (K) flag for kingside pawn storms, or kingside heavy piece attacks, or even for endgames with all pawns on the kingside.
More information about use of the Tactics flag can be found in