Tauck has different lists that it maintains to keep track of customers interests. Two such lists that I'm aware of are waitlists and interest lists.
Waitlists are associated with tours that are currently "Sold Out" in some aspect, while interest lists are on tours for which tour dates have not yet been published. I'm on an interest list for 2022 Egypt: Jewels of the Nile tours.
Another interesting aspect of waitlists is that they are different for tours that have a ship component to them vs tours that are entirely land based. The reason for this is that tours with a ship component have a multitude of types of waitlists: people waiting to get on the tour in any ship's room, people that are waiting for the tour if they can get a specific type of room, people already on the tour waiting/hoping for a different category of room, ... This is why there is no real 1 to n waitlist for these tours. Tauck has an entire department just for processing waitlists (I was told this by a representative). Tours, with ship based components, "Sell Out" by room category. Once all rooms are booked then the tour is "Sold Out".
Whereas for entirely land based tours the waitlist is strictly a 1 to n list of people trying to get on the tour.