Halliday & Matthiessen (1999: 42-3):
If we use systemic representation to encode information in the ideation base, we factor out the logic of subsumption and alternatives from the syntagmatic realisation: this logic is represented in the system network. If each feature is interpreted as a type, this gives us a lattice of types. In European structuralist terms, the network is a representation of the paradigmatic organisation of a linguistic system. However, it is also necessary to show how feature choices are realised structurally; i.e., what are the structural properties of the types. This is the province of the realisation statement.
A realisation statement is a minimal specification of a piece of structure or configuration of roles presented in a paradigmatic context; it is always associated with a particular systemic feature. For instance, the realisation statement '+ Senser' occurs in the context of 'sensing' in the system of figures; it is the syntagmatic realisation of that feature: a figure of sensing is a configuration of roles one of which is Senser.