Halliday & Matthiessen (2014: 666):
There are two fundamental semantic domains of this kind – expansion and projection. For example, the expanding relation of ‘addition’ may be realised (1) cohesively by a conjunction such as also or (2) structurally by (a) an additive paratactic clause nexus marked by the structural conjunction and, (b) a circumstance of accompaniment marked by the preposition with or (c) an additive paratactic group nexus marked by and:
(1) She went to the market. Her son also went to the market.
(2a) She went to the market and so did her son.
(2b) She went to the market with her son.
(2c) She and her son went to the market.
These realisational variants are dispersed in the grammar, since they constitute different grammatical environments; but they are semantically agnate in that they all have the feature of ‘addition’. The fact that they are semantically agnate does not mean that they are synonymous; they share the feature ‘addition’ but they differ in other respects. For example, (2b) and (2c) construe one event, whereas (2a) and (1) imply two events. One semantic system, the system of EXPANSION, has thus evolved to bring together patterns of wording within grammatically distinct units, thereby extending its overall meaning potential.