Mirror Entities in solidworks is used to replicate / mirror any features or parts / bodies w.r.t to a plane / face of a part. To demonstrate how to use this feature create a box of 10x10x5 at a distance of 30 mm from the origin on top plane as shown.
Use extrude feature to create the box.
Now in the features toolbar select mirror entities
Select the right plane for Mirror Face / Plane. Select the Boss-Extrude1 in Bodies to mirror (hover over the feature & click to select). Uncheck Merge Solids & Knit Surfaces option. You will not be able to create a mirror using features to mirror since it is used only to create a mirror of features within a body. this is demonstrated in late stage of this tutorial
Click ok to see the mirrored body..
Now create a circular extruded cut of 2mm in the left box as shown in figure.
Create a reference plane from the left face at a distance of 5mm as shown.
Select the mirror entities again and select extruded cut as features to mirror and plane1 and as mirror plane / face.
Click ok and you will see the extruded cut mirrored w.r.t to the plane we have created.
One might think why not use linear pattern instead of mirror entities? In this case since the feature is symmetric we will not see any difference in both approaches. But if you the feature to be mirrored is asymmetric, the difference is quite obvious. As an exercise try to create the following cut on the other box.
Thanks guys