Parts list files contain custom blocks used to implement custom parts lists.
The path to the folder containing parts list files is in the system editor key Acad/MPathsDwgMask (Configuration file settings gef@Paths@DwgMask).
By default, the key points to C:\Users\Public\Public Documents\Autodesk\AutoCAD Mechanical 2023\Acadm\Gen\Dwg\Msk