Register: Generalised Analysis Vs Specialised Description

Halliday & Matthiessen (2014: 54-5):

Analysis and description thus operate at the outer poles of the cline of instantiation within a given language. Regions intermediate between these two poles can be approached in terms of either analysis or description: the account of a text type can be interpreted as a generalised analysis of a sample of texts, and the account of a register can be interpreted as specialised description of the general system; but, in either case, the account will ultimately be grounded in textual data.