Read the start of the UAC book that contains all the info regarding the different offer round...it explains it all, I will explain as much as I feel necessary to this topic.
You will be considered for your first pref, if you get an offer then you have that one offer and the others are not considered for offers. but if you fail to recieve an offer you will be considered for your second one, and so on and so on till you either recieve an offer or your prefs are exhausted.
say you have 5 prefs, and in the main round you get an offer to pref 4, in the late round you will be considered only for the prefs higher in preference, so in this case pref 1,2,3. If you have changed your mind and want to do pref 5 then you will need to change your prefs.
ACCEPT YOUR FIRST OFFER even if it is your last preference, there is no guarantee you will get another one in a later offer round. Even if you recieve your 9th pref and then in the late round you get your 1st, you can still pull out of your acceptance to the 9th one and take the 1st one.
The key here is TAKE YOUR FIRST OFFER. take and grasp it tightly, if you don't you may miss out altogether.
Christopher Fife.