Halliday & Matthiessen (1999: 41-2):
Systemic representations include (i) system networks and (ii) realisation statements.
(i) A system network is an acyclic directed graph, consisting of systems partially ordered in delicacy. Each system constitutes a choice (alternation, opposition) between two or more terms. These terms are represented by features, and a system as a whole is a Boolean combination of features:
(1) It has an entry condition, the condition under which the systemic choice is available. The entry condition may be a single feature or a complex of features, conjunct and/ or disjunct. These features must serve as terms in other systems.
(2) It has a set of terms, the options that are available given the entry condition. The terms are represented by features, which are related by exclusive disjunction. Collectively, a set of related systems form a system network (since features in the entry conditions to systems must be terms in other systems).