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== XML layout ==
The files containing the layout and scripts for the game user interface can be found in Blizzard Interface Data\FrameXML.
The FrameXML.toc file is a plain text file and contains a list of Lua and XML files to load at login. The files are loaded in the order they're listed and any load errors are written to FrameXML.log in the game's Log folder.
In your own addons you will also have a .toc file which performs the same function for your own code. You will list your Lua and XML files in it and they will be loaded during login. Errors which occur while loading your addon will also be written to the FrameXML.log file.
Each XML file describes a collection of UI elements and Blizzard separates their XML files into functional modules. You will find files for the GameTooltip, ActionBarFrame, ReadyCheck, and everything else in the built-in UI. The Blizzard XML and Lua files make a great reference for understanding how to develop an addon.
The XML files strictly adhere to the XML standard and if your text editor supports XML syntax checking, the file UI.xsd in the FrameXML folder contains the schema used by the WoW interface.
Lots more details here: [[XML User Interface]].