Mirror groups are resources that are available for use throughout the project and can be managed via the Resource Manager. You can use a mirror group:
In BlockSim: To place the same block in more than one location (i.e., to represent a single component more than once in a diagram or in multiple diagrams within a project). See Mirroring (Using Blocks in Multiple Locations) in the BlockSim documentation.
In the FMRA in BlockSim, XFMEA, RCM++ or RBI: To place the same cause in more than one location in the FMRA hierarchy, in order to consider common cause failures. See Using Mirror Groups in an FMRA in the XFMEA/RCM++/RBI documentation.