Wedgie Simulator Developer Wiki
A sandbox game about giving your mannequin a wedgie.
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The visual framework of UI that displays on your screen. Such mod can be extremely malleable for special game modes and total conversions.

Similar to the P-Manné blueprint path, and unlike the rest of the mod types, UI mods require a special setup process and this is likely to be done via total conversion.
Your vehicle mod should provide you with a widget blueprint (for artistic visualization and limited programming capability), and a regular blueprint class (for full, extended programming) to guide you.
Refer to the UDN documentation and UE4 tutorials for further assistance.