%APPDATA%\Microsoft\UProof

CUSTOM.DIC is the custom dictionary file.

This file contains with one entry per line.

The U in the directory name stands for Unicode. This is a hint that such a dictionary file is a UTF-16 encoded file.

See also RoamingCustom.dic and HKEY_CURRENT_USER\Software\Microsoft\Shared Tools\Proofing Tools\1.0\Custom Dictionaries

%APPDATA%\Microsoft\Spelling

The parent directory %APPDATA%\Microsoft