When creating stencils in one of the 3 formats that Visio uses it can be very important to convert between these 3 formats quickly and easily, ok, there is a Save As command on the stencil toolbar, but what if you have 100 stencils to convert?
Load up your stencils into Visio and use this command to mass save in either VSS or VSSX format. I'm not providing a save in VSX format as that is now dead for Visio 2016 onwards.
Warning. This will save all open stencils in all open documents into the stencil's folder - ensure you don't overwrite your own or Visio's built-i stencils.