Performing a mirror operation needs a mirror line or a plane.
Frequently there is not such a mirror line / plane but there may be lines or faces already there where the requested mirroring is symmetric to. Consider for example a rectangle or a cuboid
So there may be a second option called "Mirror symmetric" that requests the selection of 2 lines or 2 faces.
Once selected, a construction line or a construction plane is generated automatically and the mirror command is performed accordingly.
So then there is no need to make a symmetric construction plane or construction(mid) line ahead of the mirror operation.
Can't find what you're looking for? Ask the community or share your knowledge.