As is almost always the case, when you can't find something, go back to the definitions. Here's the key piece of the definition of "overdraft service" under §205.17(a):
For purposes of this section, the term “overdraft service” means a service under which a financial institution assesses a fee or charge on a consumer’s account held by the institution for paying a transaction (including a check or other item) when the consumer has insufficient or unavailable funds in the account.