Serial Number
Assignment Lock 
Use
As of Release 4.6C, it is possible for several users to simultaneously create or assign serial numbers for the same material.
Note
If the lock is deactivated, the system immediately updates the serial numbers created during internal number assignment to the database.
This causes gaps in the serial numbers created, as soon as you cancel the creation operation not saved . You can fill these gaps at any time simply by creating the missing serial numbers manually.
Effects on Customizing
In order to deactivate the lock, choose Plant Maintenance and Customer Service -> Master Data in Plant Maintenance and Customer Service -> Technical Objects -> Serial Number Management -> Deactivate Lock for Internal Assignment of Serial Numbers .