The Lock Stencil feature protects a Stencil from being overwritten by updates from D-Tools. If you have modified any of the stock Visio Stencils you may want to lock them.
In the D-Tools SI window, click the Shapes tab then click on the Stencil you want to lock. Right-click and choose "Lock Stencil."
The following prompt will open, and click Yes.
The Stencil will now display in the Shape Tree with a lock icon next to it.
To unlock a Stencil, right-click the Stencil and select "Unlock Stencil."