A text is cohesive when the elements are tied together and considered meaningful to the reader

Substitution

Substitution as another type of cohesive relation, or cohesive tie, is the process in which one item within a text or discourse is replaced by another (cf. HALLIDAY & HASAN 1994:88). While reference was a relation on the semantic level, i.e. between meanings, substitution is a relation on the lexicogrammatical level (level of grammar and vocabulary) “between linguistic items, such as words or phrases” (HALLIDAY & HASAN 1994: 89). A substitute, in its broadest sense, can be seen as “a sort of counter which is used in place of the repetition of a particular item” (HALLIDAY & HASAN 1994:89). Example 7 shows this cohesive relation in which “one” substitutes the word “car”.
(7) Jack’s car is very old and ugly. He should get a nicer one.
The difference between reference and substitution is that the substituted items are always exchangeable by the items they stand for. With reference the presupposed items can almost never replace the items which refer to them. The table below illustrates this. While in (7) “one” could easily be replaced by “car” without changing the meaning of the sentence (cf. (7´)), “it” in (6) could never be exchanged by “watch” (cf. (6´)). The same is true for (4) in which “he” is not exchangeable by “John” without creating ambiguity. The reader cannot be sure anymore if the “John” in the second sentence is the same person that occurs in the first sentence (cf. (4´)).
(4´) John goes fishing every other week. John is a very good fisherman.
(6´) *There watch is, my so much admired watch.
(7´) Jack’s car is very old and ugly. He should get a nicer car.
Consequently, “the substitute item has the same structural function as that for which it substitutes” (HALLIDAY & HASAN 1994: 89). There are also different types of substitution which are called nominal substitution (replacement of a noun by “one, ones, same”, as illustrated in (7), verbal substitution (replacement of a verb by “do”) and clausal substitution (replacement of a clause by “so, not”) (HALLIDAY & HASAN 1994: 90f.).