Register Variation: The Resetting Of Lexicogrammatical and Semantic System Probabilities

Halliday & Matthiessen (1999: 554) 

We can in fact define register variation as the resetting of probabilities in the lexicogrammatical and semantic systems …. 

Halliday & Matthiessen (1999: 557) 

… we define register variation as the ongoing resetting of probabilities in the lexicogrammar, which then functions to construe the ongoing variation at the level of the social process.