Form files are the files used by the AMCOMP2D, AMEXT2D, AMOR2D, and AMBELL2D commands to insert forms for springs in a drawing.
The path to the folder containing form files is in the system editor key Acad/MPathsFormPath, (Configuration setting gen@Paths@FormPath).
By default, the key points to C:\Users\Public\Public Documents\Autodesk\AutoCAD Mechanical 2025\Acadm\Gen\Form