Notre Dame has an agreement with the ACC that includes access to its bowl slots when not in the playoffs. In exchange, Notre Dame agrees to accept an invitation when it qualifies for one of the ACC's slots.
In turn, the ACC negotiates its agreements with various bowls with the tacit understanding that getting Notre Dame is part of the deal. As a result, the conference gets better better bowls because all of them know that they wil sell more tickets and achieve higher TV ratings whenever they happen to get Notre Dame.
Everyone does well in this arrangement - Notre Dame with more access to bowl, the ACC with more and better bowl contracts, and the bowls that have ACC agreements with occasional appearances by the team that gets them more revenue than any other in the sport; but the downside of ND's contract with the ACC is that it does not have the option to decline any more than full ACC member teams have the option to decline.
When it comes to bowl games, Notre Dame is an ACC member and not an independent. This isn't new. The last contract ND had with the Big East worked the same way.