Social interactions between individuals do not occur in a void. Nor do they take place on a pre-existing fixed social network. Real social behaviour can be understood both to take place on, and to bring about, a complex set of overlapping topologies best described by a multilayer network in which different layers indicate different modes of interaction. Here we distinguish between the milieu within which social organisation is embedded and the transactional relationships that constitute this social organisation. While both can be represented by network structures, their topologies will not necessarily be the same. Researchers in various domains have realised the importance of the context in which individuals are embedded in shaping properties of the functional transactions in which they choose to engage. We review several examples of the relationship between milieu and function and propose a conceptual framework that may help advance our understanding of how social organisation can occur as a result of self-organisation and adaptation.

Milieu and Function: Toward a Multilayer Framework for Understanding Social Networks