The document number range should be created and assigned to a document type. It is an interval of number from and to and the number could either be internally assign by the SAP system or externally assign by the user or other external system. Note that every transaction in an SAP system be it a master data or transactional data creates a document number. It is a requirement that a document number range should be created before a transaction can be posted in an SAP system.
Here’s a procedure on how to define Document Number Range for Financial Accounting:
Transaction code: FBN1
Path: IMG → Financial Accounting (New) → Financial Accounting Global Setting (New) → Document → Document Number Ranges → Define Document Number Ranges for Entry View.
The “Number Ranges for Financial Accounting Documents” appears. Document number range for Financial Accounting is created at the company code level. On the company code field enter the company code e.g. 1000.
To create document number range interval selet “Intervals” button. The “Maintain Number Range Intervals” screen appears as shown in this screen shot:
Then select “Insert Interval” button to add new number range interval. The “Insert Interval” pop-up window appears as shown:
Take a look at following fields:
Number, this is an alphanumeric 2 digit field. Enter value as identifier of the document number range you want to create.
Year, enter the year which the number range is valid until. For instance you entered 9999, the number range interval is valid until year 9999.
From Number, enter in this field the lower limit of the number range interval.
To Number, enter in this field the upper limit of the number range interval.
Current Number, default value is 0. This field will be updated for the current number used during transactons.
Ext (Externally Assigned), tick this indicator if the number is externally assigned by the user or other external system.
It is recommended that transactional document number should be internally assigned by the SAP system.
However, if it’s a document number for master data you may chose to configure the number range as externally assigned especially in the case of employees. Normally, it is more pracical to use the employees ID number as their code in the system.
Don’t forget to Save your work. 
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