Question: What are “Hot and Cold Wall?”

Answer: Playing with hot and cold walls are consider table rules. Table rules are not sanctioned by the National Mah Jongg League. Since table rules are set for this variation it is not possible to list all the rules here. One example of a “Hot Wall” (the last wall to be pushed out or a set number of remaining tiles from the last wall to be pushed out) is that all players must account for three tiles either from the discard pile, in an exposure or in their own hand. If they cannot account for three tiles and they throw the winning tile they would have to pay out the winnings for the entire table. One example of a “Cold Wall” would be, to pick their own Mah Jongg during the remaining broken wall tiles (The tiles held back after the dice were rolled and the wall split). In other words, if someone discarded your Mah Jongg tile you would not be able to call it for the win.

(See also https://mahjcon.com/mahjpedia/id53)

Source:  National Mah Jongg League Bulletin (2023)