Halliday & Matthiessen (2014: 27):
The system of language is ‘instantiated’ in the form of text. … The system is the underlying potential of a language: its potential as a meaning–making resource. … This use of ‘system’ is thus different from — although related to — its meaning as a technical term in the grammar. The system in this general sense is the totality of all the specific systems that would figure in a comprehensive network covering every stratum.