"the title, the introductory clause," but "a table of contents"

In the following paragraph:

It’s standard for a contract to consist of the front of the contract, the body of the contract—what the parties are agreeing to—and the back of the contract. The front consists of the title, the introductory clause, any recitals, and the lead-in. In longer contracts, it might include one or more of a cover sheet, a table of contents, and an index of definitions.

Why does the author use "the" for title, but "a" for cover sheet?

The elements identified by the are essential; every contract is expected to have them. The others are optional; the text says they might be included. As elements that may or may not exist, they only warrant the indefinite article.