I could only find ROZ – Research-Oktanzahl - German for Research Octane Number.
....So I'm inclined to think that RON and ROZ ratings are the same and that my fuel flap label is merely stating that the car can use either 98 (in 2005) or 95, and that more advice is written in the Handbook.
Long story short = Use 98 (minimum) whenever possible and only use 95 in emergency and granny drive accordingly.
This of course is advised and written by VW for their standard OEM cars, not ones remapped by mad Englishmen.