Every Screen has a Layout. Layouts define the Highlevel-Structure of a Screen. This means they specify the number of Panels contained in the Layout, and the way they are arranged/layouted.
Layouts are named by a Combination of a Number and a Character, where the Number defines how many Panels the Layout contains and the Character specifies how the Panels are arranged.
E.g. 2U means 2 Panels, arranged horizontally, 3E means 3 Panels arranged vertically.