I am going to highlight some important concerns about preferences. You are asked to give your preferences for area of training at some point in training application. In some specialties it is asked when you are submitting your application (GP training) for some it is when you have passed MSRA (Obgyn). Usually its before you are given your rank number.
If someone has submitted two applications for different specialties, preferences will be selected separately for each application.
You can change your preferences before you are given an offer (It gives you warning when its closing)
Please put areas of your preferences in order of your real preferred area, not based on your scores. If you
Do not want to be in an are, please do not include in your preferences.
You will be offered a place first available on your preference list according to your rank number , its not other way round. (MOST IMPORTANT POINT) If someone has rank number 1 and their preferred area is Scotland (lets say less competent for that field) they will get it there.
If you are given an offer , you can opt in for upgrades (meaning if any preferred area of your list above your offer becomes available you will be promoted. It is not downgraded.
You can not exchange your offered area with someone.
If at the time some areas do not show any slots available , it may become available later. So if you are interested in that area , just include in your preferences regardless of seats available.
In a nutshell , do not overthink and complicate. Just rank your areas in terms of actual preference.