Hi,
Purchase order seriel numbers should be based on plant wise & Document type wise
Example:
Plant Document Type Seriel number
1000 NB 4500000000 - 4599999999
1000 UB 4600000000 - 4699999999
1000 ZSUB 4700000000 - 4799999999
1100 NB 4800000000 - 4899999999
1100 UB 4900000000 - 4999999999
1100 ZSUB 4400000000 - 4499999999