iMtG U is a version of the app intended for mass education oriented institutions such as schools.
Instead of In App Purchases the app functionality is unlocked at the moment of purchase from Apple Store, and offers significant discounts* for purchases of 20 and more copies.
The Shop entry is hidden from Main Menu in this version of the app. Other than that, U version of the app functionality is the same as iMtG Plus with I'll Have it All purchase unlocked.
* 50% as of July 2016.