Halliday & Matthiessen (2014: 444):
Within the general categories of expansion and projection, we recognise first of all a small number of subtypes: three of expansion, and two of projection. The names of these, with suggested notation, are as follows:
(1) Expansion:(a) elaborating = (‘equals’)
(b) extending + (‘is added to’)
(c) enhancing x (‘is multiplied by’)
(2) Projection:(a) locution “ (double quotes)
(b) idea ‘ (single quotes)