There is only one file type that uses the .VSDM extension:
Visio Macro-Enabled Drawing
Macro-enabled drawing created with Visio, a technical drawing program; serves the same purpose as a .VSDX file, except it allows VBA (Visual Basic for Applications) macros to be stored and executed when the drawing is opened.
Visio macros are used to automate repetitive drawing tasks such as setting shape sizes and positions or setting page formatting options. They may be written from scratch with VBA source code or recorded as a series of user actions ().
NOTE: The VSDM format, as well as the VSDX format, were introduced with Visio 2013. Previous versions of the software used .VSD and .VDX files for drawings.
Updated: December 26, 2012