You can define the Material Master Number Range in transaction MMNR.

Click the button Groups to Maintain the Material Type Grouping.

Below are those material type that have no number range groups. 

To assign a group, in the not assigned group, select the Material Type you want to group.

Then tick the Group you want to assign and click the menu path Edit -> Assign Element Group.

From the same screen, if you want a new number range intervals, click Group -> Insert Group.

If you decided to used an external number range, goto transaction OMS2 and 
tick the field External no. assignment w/o check.
 

You have two ways of assigning number range :

Internal number assignment 
In this case, a number within the number range interval allowed is assigned by the SAP R/3 System.

External number assignment 
Here, the user assigns a number within the number range interval allowed. You can define the intervals for external number assignment numerically and alphanumerically.
 

You can assign one or more material types to each group.

If you assign just one material type to a group, you can define both an internal and an external number range interval for the material type. 

If you group together more than one material type, you can define an internal and an external number range interval here too. This allows several material types to have the same number range intervals. 

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