"Single Player Deathmatch" (g_gametype 2) is the gametype used when playing locally (Singleplayer menu) and selecting the predefined matches against bots (not those in the Skirmish menu). It's also referred as "Unlock tiers" mode or "Campaign" mode. After winning one of these matches, a symbol will appear on its icon, remembering the highest difficulty level you have been able to beat that challenge. Once all of the four maps are won regardless of their difficulty level, a newer tier will unlock, and you will be able to play four more matches, until you reach the final challenge.