The EQ2 UI still uses eq2ui_mainhud.xml. However, eq2ui_custom.xml was added so that custom UI authors could use that for their files and so that the game would thusly always draw from the default eq2ui_mainhud.xml.
Not all authors have switched to a system using just eq2ui_custom.xml to contain their files, and even for those that have old files may be floating about causing issues. But the file itself is still used.
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