Question: Can you explain how the White Dragon works when it is a Zero and why was this tile chosen to be a Zero?

Answer: The White Dragon (Soap) was chosen to represent a zero because originally they were all blank. The blank tiles were equivalent to nothing and considered a zero. This tile is used whenever a year hand includes a Zero. For example, in 2023, the Zero would be represented by the White Dragon (Soap) tile. Today, many of these tiles have rectangular designs on them. This ruling is not found in Mah Jongg Made Easy.

Source: Mah Jongg Made Easy 2024, page 6 & the National Mah Jongg League Bulletin Q&A (1981 page 5, 2000 page 5)