I am not sure how it is now, but in the past and from my experience: house seats used to be held until 24 hours before the performance; that went to 48 hours; then 72 hours. Now, I think it’s 96 hours. The were released at 6:00 the day before the performance. They are sold at regular box office prices, not premium. They are contractual: each person who is eligible gets an agreed upon number of pairs. And, as far as I know, there is always an emergency pair. Of course, this may have all changed.