A. In UDT, Universe are created with file extension as .unv. In IDT, Universe file extension is changed to .unx file
B. To open unv file in IDT, it can be directly opened but you can convert unv file to unx file to open in Information Design tool & You can open an IDT .unx file in Universe Design tool nor you can convert it to unv file
D. Universe Design tool is single source enabled however IDT is multi source enabled means you can extract the data from different data sources while creating a Universe