Using classification definitions allows you to assign three-dimensional AEC objects as different object types. This is helpful if you want to export the object in your drawing as something different for the IFC file. For example, you can draw a mass element, and classify it as a specific object type.
When you work in the AutoCAD MEP 2022 toolset environment, any classification applied to AutoCAD MEP toolset objects are ignored during export as they are exported intelligently to their associated IFC entities.
A drawing file of classifications (that can be used for objects at export) is available in the software and can be found in ProgramData\Autodesk\ACA <version>\enu\Styles\IfcPropertySet_Definitions.dwg.