Document Numbering 
When creating a business partner, item, or document in SAP Business One, a number is assigned to each business partner, item, or document. This number is used as a reference and can be used as a selection criterion in various reports. Each company has its own numbering method and rules. The document numbering function lets you define various numbering series and determine the default numbering series for the master data and documents.
SAP Business One provides two numbering methods:
Separate numbering series for each type of master data or document
This method is the default for all localizations except Belgium when a new database or company is created. Unless a document is entered, or a business partner master data is used in any way, an authorized user can change the setting to a model that allows numbering series to be used for more than one type of master data or document.
One numbering series used for more than one type of master data or document
According to this method, a specific numbering series can be assigned to more than one type of master data or document. This method is the default for the Belgium localization when a new database or company is created.
CautionThis numbering setup is irreversible; in other words, you cannot change it after selecting the Permit More than One Document Type per Series checkbox on the Basic Initialization tab of the Company Details window.
End of the caution.
The numbering method is defined in 
Administration 
System Initialization Document Numbering 
.