Deathmatch

Deathmatch is a game mode first introduced in v100 Alpha. As of v123 Alpha, there are no teams by default in Deathmatch (though, they can be set before starting the match).

Custom Deathmatch
Before the v123 Deathmatch revamp, deathmatch maps were played in teams and always 'using the map settings' (since there was no other option back then). The objective was still the same. No official map uses the dm_ gamemode anymore, as they have all been updated to d_. You can still install user maps that use this gamemode (their prefix is dm_ instead of d_) and the maps will still work as they did.

Custom deathmatch worked as if each character got it's own team that it was championing for points. You can still apply teams to regular deathmatch, however deathmatch will work at default as if there are no teams, and each player/character is earning their own points, for themselves only.