A show releases a certain number of tickets to TDF, let's say 25. It guarantees there will be at least 25 seats to TDF so if the show were to sell all but 25 tickets for the performance, it would pull everything and then be sold out. This gives a show a lot of flexibility since assuming there are more than 25 available, it doesn't need to decide which 25 until a few hours before the performance when seats are actually assigned. It can then try to sell premium and expensive orchestra seats up until the last minute and/or use TDF to pad areas of the house that would look bad if they were empty.
Scratch and claw for every day you're worth!
Make them drag you screaming from life, keep dreaming
You'll live forever here on earth.