Symmetry is the action of painting in multiple locations at the same time based on geometric constraints.
 For example: painting in mirror based on an axis. Symmetry can be enabled by clicking on the Symmetry button in the Contextual Toolbar. Symmetry is only available when using a Painting Tool

For more information, see the following pages:

Paint projection and stencil projection in the 2D view don't support symmetry. We recommend using the 3D view instead if needed.