THE WHITE DRAGON IS A DUAL TILE (Article 104)

Commentary: White dragons are dual-purpose tiles. They can function as dragons, corresponding to the dot suit, or as zeros (e.g., neutral; always dark blue). When used as zeros, they no longer correspond to dots and are treated solely as zeros. Zeros can used in a math play hand or a block of Year tiles (e.g., 10, 2025, 000, 0000). Consecutive runs (e.g., sequences) can only consist of number tiles ranging from 1 to 9.

Question from a player:  Why is a White Dragon used as a zero? 
 
Answer from the NMJL: The White Dragon is usually a tile with a blue border around the edge. This is often referred to as “Soap.” Only this Dragon can represent a zero. The Soap, when used as a zero, can be used with any suit. It can be used with Dots, Bams or Craks. 

Source: Mah Jongg Made Easy (2024 page 6)

Other answers from the NMJL: “Our reasoning, White Dragon is a blank tile.  Blank is nothing and nothing is zero.  When used as a zero it has no matching suit.  Only a joker can be used in its place.”  
 
Source of other answers: National Mah Jongg League Bulletin Q&A (1981 page 5, 2000 page 5)