Halliday & Matthiessen (2014: 54):
If we have access to an existing account of the system of the language (at the potential pole of the cline of instantiation), then we will analyse texts by relating instantial patterns in the system. In other words, we undertake the analysis of texts by means of the description of the system that lies behind them, identifying terms in systems and fragments of structures that are instantiated in the text. In the course of undertaking the analysis, we are likely to find gaps in the description, or even mistaken generalisations. Text analysis is a very rigorous way of testing, and thus improving, existing descriptions because everything in a given text has to be accounted for in the description.