I believe Notre Dame has a football agreement to play 5 ACC teams each year as an Independent. The benefit for Notre Dame is having 5 P4 games they don’t have to negotiate and leaves them 7 games with flexibility to play their traditional opponents. And the benefit for the ACC is 5 high profile games and additional TV money.
Notre Dame’s playoff agreement is separate from the ACC’s.
In years Notre Dame doesn't make the playoffs, they are slotted into the ACC bowl pecking order.
Most projections had them playing BYU in the PopTarts bowl in Orlando as the "ACC" rep.