Substitution

Substitution involves the replacement of one item by another in the text. Replaced items may be a word(group) or a sentence, while replacing items are PRO-FORMS such as ONE,  ONES, DO, SO. This type of substitution is a way to make texts more concise by avoiding tedious repetitions<<<<<<<<<<.
"My axe is too blunt. I must get a sharper ONE"
"Is it going to rain? I think SO".
One: replaces nouns
Do: it is used to replace verbs
So: it is more often used to replace whole clauses.


An important point is the difference between Substitution and Reference. According to Halliday and Hasan, substitution is a relation in the wording, while reference is a relation in the meaning. Reference is placed on the semantic level, while substitution is a relation on the lexico-grammatical level.
Substitution is a relation within a text: a substitute is used in place of the repetition of a particular item. The substitute item has the same structural function as that for which it substitutes.
Axe and one are both head in the nominal group: My axe is too blunt. I must get a sharper one.
Does and know are noth head in the verbal group: You think John already knows? I think everybody does.

In reference, there is no such restriction, and the structural function of a reference item MAY be quite different from that of its referens: If our commanders say we need troops, I will send THEM.