"Width in number of modular spacings [8EF002950]" definition

The definition for what is classed as a "modular spacing" is unclear, is anyone able to help define/explain the concept? We have a modular product, e.g. a four unit product I would expect to have a value of "4" in this instance, however predecessors have filled the value as 0,2,3 etc. 

Modular spacing refers to a standardized size where each module is 18 mm. I am sure this according to a certain standard, but can't find it easily. But it probably is a European standard. See attached screenshot for a better understanding. Hope that someone can refer to the correct standard or where the 18 mm is else defined.