Halliday & Matthiessen (2014: 56):
This introduction to (systemic) functional grammar differs in various ways from other accounts – in terms of both theory and description. (i) In terms of theory, we can locate systemic functional theory of grammar within a general family of functional theories of grammar, contrasting these with formal theories of grammar. Within the family of functional theories, systemic functional theory is unique in its paradigmatic orientation – its orientation to grammar as system, represented by means of system networks; other functional theories are syntagmatic in their orientation. Systemic functional theory also differs from many other functional theories in its emphasis on comprehensive, text-based descriptions – descriptions that can be used in text analysis; other functional theories have tended to foreground linguistic comparison and typology based on descriptive fragments from a wide range of languages.