Halliday & Matthiessen (2014: 642-3):
The types of cohesion we have discussed so far all involve grammatical resources – grammatical items (conjunctions, reference items, substitute items) and grammatical structure (absence or substitution of elements of structure). However, cohesion also operates within the lexical zone of lexicogrammar. Here a speaker or writer creates cohesion in discourse through the choice of lexical items. … lexical cohesion comes about through the selection of items that are related in some way to those that have gone before.Just as ellipsis and substitution take advantage of the patterns inherent in grammatical structure (ellipting and substituting particular elements of structure such as the Head of a nominal group), so lexical cohesion takes advantage of the patterns inherent in the organisation of lexis. Lexis is organised into a network of lexical relations such as the ‘kind of’ relations obtaining between fish and salmon.