UniversalForm is defined in WorkshopFramework, in the script WorkshopFramework:Library:DataStructures.
The purpose of this is to allow for using forms from external plugins, or from the current plugin within the same script property, the form from this structure can be fetched using the function: WorkshopFramework:Library:UtilityFunctions.GetUniversalForm, which will handle loading the form from the external plugin when necessary.