I am struggling to find what the ITUNMOVI for a specific song is. I have read previous posts which said that the value for ITUNMOVI, which is higlighted below, (10200433) was the size of the file but i found out that it's not true because I checked that value and the size of a file which already had the ITUNMOVI field filled, and they did not match. Would someone be able to tell me what that value indicates and how to work it out? Thanks.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "">
<plist version="1.0">
[/u]        <key>flavor</key>

see e.g.

I'm relatively new to Mp3Tag so i dont understand what all of that means but from what i gathere they're talking about adding that field to a movie and the process should be different.

To get this all clear: this is the forum for the application "MP3tag".
It deals with the program features and how to use them to tag files that are supported by the program.
This forum is not about the contents of individual fields unless they follow a certain syntax that has to be obeyed.
To me ITUNMOVI looks like a field (atom) that originates with iTunes and therefore any semantics concerning this fields should be clarified with an iTunes related community.
Yet, if you are lucky and a member of this forums is able to help you: fine.
But as far as I can see the thread /t/14757/1 did not satisfy you and not many have been added since (and you hardly find anything elsewhere on the net) you many not get the exhausting information that you are looking for.