Line numbering of master detail reports

In addition to the Line system variable, a LineThrough system variable has been added for numbering master-detail lists. What is the difference? The LineThrough system variable is used to output numbers using continuous numbering. Below is the same report numbered consecutively.

In this case, the numbering of the detail listing starts before the first line of the detail listing, not after the lines of the master listing are printed. System variables start numbering from 1.