Halliday & Matthiessen (2014: 49):
Giving priority to the view ‘from above’ means that the organising principle adopted is one of system: the grammar is seen as a network of interrelated meaningful choices. In other words, the dominant axis is the paradigmatic one: the fundamental components of the grammar are sets of mutually defining contrastive features. Explaining something consists not of stating how it is structured but in showing how it is related to other things: its pattern of systemic relationships, or agnateness (agnation).