This is the classic 'rule-oriented' sub-trait, relating to a person who need to be absolutely sure of their position and prefers to use established regulations and procedures as a framework to support their ideas. Co-operative people are so called because this aspect of their personal style extends to the need for practical support from managers, colleagues and friends, and so they seek to maintain positive working relationships with others.
Co-operativeness is sometimes mistaken for a social sub-trait, but this is not the case. People of this kind work co-operatively with others as part of a behavioural strategy: it allows them to avoid individual risk and distribute responsibility. (This is not to say that there will not be other social drives in the behaviour - whether or not these are present will depend largely on the levels of Influence and Steadiness present in the profile).
Regardless of their motivations, though, Co-operative styles tend to be good team-workers. They are able to focus on the good of the group as a whole rather than on their own specific needs, and the organised, structured style that commonly accompanies this sub-trait can also be a strength in a group situation.