Difference between internal and external number ranges in SAP?

In case of Internal number ranges,system will assign the numbers based on the number interval you have assigned in FBN1.

IN case of external number range, the user has to manually assign the numbers based on the number interval youn assign in FBN1.

To have the external number range, select the check box external in FBN1 when you definr number range intervals in FBN1.