The boarding card says that I HAVE a reserved seat - but no details as to what it might be.
You do in deed have a reserved seat or you will be sitting on the wing.
Without meaning to sound condescending, you really should of checked what this entitled you to, before paying for the service.
All reserved seating gives you, is a guaranteed seat in the first three rows of the aircraft, to allow you to board and disembark quickly. It does explain this during the booking process.
WIZZ Reserved Seat including priority boarding
I haven't travelled with the airline in the last few months, but as far as I am aware Wizzair is one of the very few remaining carriers, to have a free seating policy, so can sit where you want! Such a ashame other airlines don;t offer this.
This probably applies to the rows you are booked to fly in, no guarantees which seat you have, will be a free for all, with all 18 passengers booked to use these seats.