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Locking Material Master Records 
Use
As of Release 4.6C, you can lock a material master record. This means that the fields you have defined in Customizing as lock-relevant can no longer be changed in material master maintenance. In dialog mode, these fields are not ready for input. (This does not apply to required fields in which an entry has not yet been made when you extend a material master record.)
You can lock a material master record only if there are no scheduled changes to lock-relevant fields. The lock takes effect when the material master record is saved.
Once locked, a material master record cannot normally be unlocked. However, in exceptional cases, a material can be unlocked with a special authorization, provided the material is not used in configuration management.
Effects on Customizing
You define which fields are lock-relevant in
Customizing for the Material Master in 
Define Lock-Relevant Fields.
See also
For more information (including the authorizations necessary), see the SAP library documentation Material Master (LO-MD-MM) -> Locking Material Master Records.