Structural units of this kind are most often used to identify specific locations or reference points within a text (``the third sentence of the second paragraph in chapter ten''; ``canto 10, line 1234''; ``page 412,'' etc.) but they may also be used to subdivide a text into meaningful fragments for analytic purposes (``is the average sentence length of section 2 different from that of section 5?'' ``how many paragraphs separate each occurrence of the word `nature'?'' ``how many pages?''). Other structural units are more clearly analytic, in that they characterize a section of a text. A dramatic text might regard each speech by a different character as a unit of one kind, and stage directions or pieces of action as units of another kind. Such an analysis is less useful for locating parts of the text (``the 93rd speech by Horatio in Act 2'') than for facilitating comparisons between the words used by one character and those of another, or those used by the same character at different points of the play.
In a prose text one might similarly wish to regard as units of different types passages in direct or indirect speech, passages employing different stylistic registers (narrative, polemic, commentary, argument, etc.), passages of different authorship and so forth. And for certain types of analysis (most notably textual criticism) the physical appearance of one particular printed or manuscript source may be of importance: paradoxically, one may wish to use descriptive markup to describe presentational features such as typeface, line breaks, use of white space and so forth.
These textual structures overlap with each other in complex and unpredictable ways. Particularly when dealing with texts as instantiated by paper technology, the reader needs to be aware of both the physical organization of the book and the logical structure of the work it contains. Many great works (Sterne's Tristram Shandy for example) cannot be fully appreciated without an awareness of the interplay between narrative units (such as chapters or paragraphs) and page divisions. For many types of research, it is the interplay between different levels of analysis which is crucial: the extent to which syntactic structure and narrative structure mesh, or fail to mesh, for example, or the extent to which phonological structures reflect morphology.