From 6e2 page 92:
+1 DAMAGE CLASS
Characters may improve their Martial Maneuvers by buying Extra Damage Classes. Each Extra DC adds one Damage Class to any Maneuver (such as Martial Strike, Killing Strike, or Martial Throw), or +5 STR to any Maneuver that grants extra STR for limited purposes (such as Martial Grab).
An Extra Damage Class adds to all of a character’s Martial Maneuvers — he doesn’t have to purchase Extra DCs maneuver by maneuver. Characters cannot purchase half an Extra DC, nor apply Limitations to Extra DCs. A character can choose not to use his Extra DCs if he wants to do less damage than normal.
From CC page 155:
EXTRA DAMAGE CLASS
Each Extra Damage Class adds one DC to any Martial Maneuver, or +5 STR to any Martial Maneuver that grants extra STR for limited purposes. An Extra Damage Class adds to all of a character’s Martial Maneuvers.