The purpose of the table of contents is to name the different parts of the document, and clues for the indexer are less explicit and require a different analysis than that of the title
The table of contents will never directly state the subject matter of the document but simply name some of the parts of the document,
since the purpose of the table of contents is to guide the reader through the book.
The indexer needs to put the parts together, and, as such, the subject description is as much a product of the indexer as of the document.
The indexer needs to make a decision about how to represent the document and, again, this decision needs to be based on information other than that which the indexer can gather from the document