The Mirror button allows to mirror a selected entity or entities relative to a given axis.
Quick Access Panel: 
Ribbon: Editor -> Instruments -> Mirror
Menu: Tools -> Modify -> Mirror
Select an entity and press the Mirror button.
Example:
•select an entity or a group of entities:
•press Mirror button;
•left mouse button click to set first mirror axis point:
•moving mouse cursor to get the necessary placement for the selected entity(ies) or use the dynamic input;
•left mouse button click to fix entity(ies) at the new placement:
Note: selected entity(ies) are copied if Fast Entities Input 
mode is on:
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