Halliday & Matthiessen (2014: 443):
(1) Expansion: the secondary clause expands the primary clause by (a) elaborating it [= ‘equals’], (b) extending it [+ ‘is added to’] or (c) enhancing it [x ‘is multiplied by’].(2) Projection: the secondary clause is projected through the primary clause, which instates it as (a) a locution [“ double quotes] or (b) an idea [‘ single quotes].Expansion relates phenomena as being of the same order of experience, while projection relates phenomena to phenomena of a higher order of experience (semiotic phenomena — what people say and think).