The EQUI route, which is what I used myself on several UIs (many of which never got posted on eqinterface).
Code:
<XML ID = "EQInterfaceDefinitionLanguage">
<Composite>
<!-- Grab the default EQUI.xml to avoid breaking the UI every patch -->
<Include>../default/EQUI.xml</Include>
<!-- Load up Eidetic's resources -->
<Include>MyUI_Textures.xml</Include>
<Include>MyUI_Animations.xml</Include>
<Include>MyUI_Templates.xml</Include>
</Composite>
<Schema xmlns = "EverQuestData" xmlns:dt = "EverQuestDataTypes"/>
</XML>
I think an explanation is much more understandable though. The Composite tag tells EQ your going to load in some more files for it to parse. Include tags tell it the path to the file.
When your UI loads the path is whatever folder your UI is inside of, so to get back out of that you must use the '../' operator which universally understood as "go back one directory". You could have it go back as many directories as you want, but it's only safe to go back as far as the EverQuest directory "../../" since you have no idea what the directory structure looks like on a user's machine beyond that and could wind up crashing EQ due to invalid file permissions trying to load stuff outside of it's directory.
When EQ reads in default/EQUI.xml it'll encounter another Composite block and attempt to load every file that block tells it, as well as any other Composite blocks it finds into those files (or any other file you pull in).