Direct answer
A modification is any departure from the light vehicle as it was certified. Like-for-like replacement of a worn part with an equivalent is usually not treated as a significant modification. Changing performance, geometry, emissions equipment, lighting type, seating or the description on the register is. VSB 14 and your state agency draw the operational line. This note does not replace either document.
Why the word matters
People use “mod” to mean a sticker. Inspectors use it to mean a compliance event. If you tell an insurer the car is unmodified and the inspection photograph shows a different exhaust path, you have a disclosure problem as well as a standards problem.
Approved is still a modification
Victoria’s approved list is a list of modifications that the regulation treats as approved when they are done in the stated way. It is not a list of things that “do not count”. That distinction matters when a later owner, insurer or interstate registry asks what has been done.
Related
Legal modifications, VSB 14 explained. Check with your state regulator if you are unsure whether your job counts.