Provides a way of duplicating Regions with new joint names and replacing the Side field of the Region. Generally used to create a mirrored set of Region markup from one side of the character to another.
The mirror process will only apply to regions that are currently visible in the Region Display. For example, if you have filtered the UI to only display 'left arm' Regions, when Mirror is pressed it will only mirror the 'left arm' Regions that match the Replace field.
Replace - Region Side to search for to determine which Region's should be mirrored. Followed by the joint name text to search and replace to determine which joints should be used for the mirrored Regions.
All Regions that have a Side that contains the text entered will be replaced. Such as 'left', 'front_left', 'left_back', etc.
Replace With - Region Side to replace followed by joint name text to replace. The Tools Config 'Rigging -> JointNamePattern' can be edited to define which side of your joint names the side text can be found on.
Mirror - Mirror all regions in the Region Display that match the Replace fields and where the joints exist with the new joint names.