Every character is limited in the number of actions and reactions he/she can take in a turn. To represent this, each character receives a number of action points at the start of a combat turn. Each action or reaction he/she takes uses up some action points. When he/she runs out of action points, he/she can no longer take any more actions until the start of the next combat turn.
At the start of each turn, every character involved in a fight makes a dice roll using his/her Combat (including combat speed specialism) + CO. The result of the dice roll is the number of action points that the character has that combat turn; the minimum number of action points a character can have in a turn is three - if the dice roll result is less than three, the character receives three action points anyway.
For example, Cogan the barbarian has a Combat skill score of 6 1/2 with a +1 specialism bonus in combat speed, and a CO score of +1/2. Cogan can choose to have a guaranteed number of action points of 8, or take a risk and have D+5 or 2D+2 action points. He decides to take a risk and rolls a 6 and a 5, giving him 13 action points that turn.
If a character is mounted, his Riding skill may restrict his combat speed.