File references can be locked using a Lock option to prevent them from being accidentally modified when they are referenced in your scene. When on, the Lock option prevents edits to all attributes and nodes associated with the file reference. You can access the Lock option from the Outliner contextual menu, Create Reference Options window, and Reference Editor.
In the Outliner, an 
icon indicates that the reference is locked, while in the Reference Editor, an 
icon indicates that the reference is locked.
When using the Lock option be aware of the following:
- If you reference a file that has locked attributes, they remain locked regardless of whether or not you turn on Lock when you create the reference.
- You cannot assign reference edits to a locked reference.
- Loading and unloading references does not affect their locked state.
- You can use a MEL script to specify which attributes you want excluded from a reference's lock state. Regardless of which attributes are excluded, the Lock option always locks all of the nodes for the referenced file.
See Lock file references.
Customizing lock settings to exclude attributes
When a reference is locked, all nodes and attributes, with the exception of a few specific attributes and node types, are locked by default. You can use MEL procedures to customize the reference Lock option so that it excludes specific attributes. When excluded, these attributes can be edited.
See Exclude attributes from locked references.
For more information on working with MEL procedures in Maya, see the MEL Overview guide. For more information on customizing the lock settings in your particular production environment see below.