Unlike the structural units such as Introduction, Conclusion, etc, Content Clusters are semantically based. Macrostructural units and Content Clusters do, however, sometimes overlap.

• Macrostructural units are usually determined by the genre, providing the overall organiational norms of a particular style of written communication.
• Content clusters vary according to the subject and the importance of different sub-topics within the content area.

Content clusters are categories of information that represent topics and sub-topics. The notion of content clusters can apply across disciplines. Thus :

• a 'case study' is a cluster of information that occur in any discipline, and
• 'locality' is a cluster which might locate a particular species in one discipline or an experimental site in another. Information pertaining to a 'locality', therefore, is something that is ideally clustered together rather presented in part here and there in the text.

• the topic of a cluster is highlighted, perhaps by being explicitly introduced at the beginning of the cluster, and
• if the paragraphs within a cluster are explicitly linked with the usual textual links. (Kaldor, Herriman & Rochecouste 1998: 26).