It means the allocation for the ACO stand has been filled. However, should someone not take up their ACO stand allocation, there may be seats available. Hence you are on the waiting list (how far down is anyones guess).
But, they will also offer you an alternative, this may (or may not) be as good a stand location - it might be up by the Esses and you won't see the start / finish line.
What have you been offered?
The story is that the ACO dont't know their a**e from their elbow.
Just wait until April / May time, you will see nothing but posts on this site complaining of poor service from ACO.
Not from me. Book early (like 12 months) and be patient and courteous in your dealings with the booking office.
If you f**k them about, they'll certainly do the same to you.