The active plane is the plane that is used as a mirror plane or the planar lock plane during sculpting modes. Both of these settings are turned off by default but they can be enabled by using the 'Settings' dialog.
Setting the Active Plane
Choose Tools->Artisan->Set Active Plane from the menu.
The black rectangle shows the current active plane.
Click to set a new plane.
Hold SHIFT to lock the current plane orientation.
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