Having mapped the NX Material attribute to a Material property on the Item Revision Master Form, it is not possible to manually edit the Material attribute from the Attributes page of the "File --> Properties" dialog in NX.
Solution
You will not be able to manually edit the attribute using "File --> Properties". It is tied to the NX "System" Material attribute and (OOTB) can only be modified if you assign a different material from the Materials Library.
It is possible to setup the NX environment to allow manual edits to the Material attribute value. You can either assign the NX system Material attribute for "Part Material" and "Object Material" to a different name (via the Customer Defaults) or uncheck the Part Material and Object Material "Create Attribute" defaults.
File --> Utilities --> Customer Defaults --> Gateway --> Materials/Mass
Select the Attributes tab. Here you can enter a different name for the Attribute Title or uncheck the defaults:
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Part Material
[x] Create Attribute
Attribute Title
Material
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Object Material
[x] Create Attribute
Attribute Title
Material
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OOTB both Part Material and Object Material are checked and assigned the attribute name "Material".
Restart NX to see any changes made to the Customer Defaults.
Notes and References
Hardware/Software Configuration
Platform: INTL64
OS: window
OS Version: 764SP1
Product: NX
Application: GATEWAY
Version: V1847
Function: CUSTOMER_DEFS
Ref: 002-7006785