Special Circumstances (SC for short) is a subset of Contact that assigns itself the handling of unusual, tricky, secret, or otherwise "special" situations. Generally viewed with a mixture of contempt and admiration by the Culture at large, SC does the dirtiest and bloodiest work that the Culture (in SC's opinion) needs doing. Described by Bora Horza Gobuchul as "their [the Culture's] euphemism for Military Intelligence," SC often acts outside of the values and morality of the Culture, in order to propagate and preserve those same values.
Since the Culture has no government as such, SC is not an official agency. By common practice and custom, however, SC inserts itself into situations in which it deems a relaxation of ordinary Culture scruples is (or may become) essential. From time to time this may annoy other factions of the Culture who deal with non-predictable situations (such as the Interesting Times Gang, or very often Contact itself).
Within the Culture, it is very difficult to be admitted to Contact; they routinely turn away scores of even the best of the best of those who apply to join. Special Circumstances is even more particular: membership is by invitation only. This exclusivity combined with the often morally ambiguous or outright unsavory tactics they sometimes employ leads to a certain amount of negative feeling among average Culture citizens. SC is "the elite of the elite, in a society which abhor[s] elitism." By that same token, the life of an SC agent is often seen as glamorous, alluring, and exciting.