This setting is useful only if you have very special requirements and your localization allow it. By normal users, it should be avoided in any possible work-around instead of this function. In simple word, if you could ignore any number systems and don't care what is what, then use it.
Certain country regulations permit document numbering series that contain more than one document type.
All of these documents are part of the same numbering series.
Select this option if these regulations are relevant for the company.
This setting determines the settings that are displayed in certain Document Numbering windows,
and cannot be changed after it has been selected.