Direct answer
The Australian Design Rule compliance plate (or the later equivalent identification) records how the vehicle was certified for this market, including a date of manufacture that many modification pathways use. Photograph it before you plan an engine swap, a seating change or a lighting conversion. The ADR index is the public catalogue of the rules themselves.
Why modifiers care
VSB 14 work is about keeping or restoring compliance. You cannot restore a rule you have not identified. The plate is the start of that identification, together with the VIN and the registration certificate.