At some point in these five years I found out that those options can be used to choose the overall size of the layout relative to the screen size.
Fill Screen = the layout will always be the size of the screen it’s on.
Custom = you can specify a size for the layout regardless of the screen it’s on.
Custom on Screen = same but there is an additional outline for the screen size.
Desired = layout is large just enough to fit all of the widgets within it while still maintaining the spacing set by the Anchors.
Desired on Screen = same but you can also see the edges of the screen
Hope it helps!
EDIT: a couple of helpful links when starting out with umg resolutions and scaling: