Short overview: all <Name /> fields are normally optional, if you click them it will jump to the area where that element is defined.
Most parameters are optional as well. if the contain values ,example: start="0" ,then that is meant as their default value. (false=default bool)
If they contain a name such as int,float,string,etc. it's meant they have no default ,but need the value to be in that format.
I created it by checking UI.xsd of WoW by hand ,and made a more clear structure in XML out of it, so you can better read it to understand it.
This information is more for the Advanced Addon Programmer ,i will add more help for the Beginners later on.