This paper focuses on the problem of generating coalition structures for task allocation via coalition formation. It provides a unified formal framework for constructing those coalitions structures. The framework takes as input a set of coalitions whose structures are abstract, a conflict relation between the coalitions, and a preference relation between the coalitions and returns the coalitions structures. Three semantics for coalitions structures will be proposed: a basic semantics which will return a unique coalition structure, stable semantics and preferred semantics. These two last may return several coalitions structures at the same time. Categories and Subject Descriptors I.2.11 [Distributed Artificial Intelligence]: Coherence and coordination General Terms Human Factors, Theory Keywords Coalition formation