yr object mode
The Real Mirror tool is used to turn the results of the mirror modifer into real geometry.
This is done for two main reasons:
- To get a proper origin for the mirrored part, so object space texturing works as expected.
- To mirror custom normals,
which the mirror modifier can't do1.
One or multiple objects, with one or multiple mirror modifiers, that are visible and renderable and have at least one mirror axis checked.
Using Real Mirror
Blender's mirror modifier has received support for custom normals some time after 2.80 ↩