There is only one file type that uses the .VSSM extension:
Visio Macro-Enabled Stencil File
File used by Visio, a program used for making business diagrams; stores a collection of shapes that can be used for constructing diagrams; serves the same purpose as a .VSSX file, except it enables macros as part of the package.
Since stencils contain custom shapes, sometimes it also makes sense to include macros that help to format and place the shapes within a diagram. Therefore, organizations may provide VSSM files instead of VSSX files when distributing their business stencils.
Updated: November 26, 2012