When you book a select fare, you have the ability to choose your preferred dining option.
Apart from Iona and Arriva you have three options - anytime dining (what it says on the can and the most popular- you go to the dining room when you are hungry) - early fixed dining (normally around 18:00, and you will have the same table and dining companions the whole cruise) -late fixed dining (same as early fixed dining, but later!)
As you are on Iona, I don't think priority dining has any relevance whatsoever.