In many iPad apps—including Mail, Messages, Notes, and Files—you can open an item in the center of the app window. To do this, do any of the following:
° Touch and hold a message in a mailbox in Mail.
° Touch and hold a conversation in Messages.
° Touch and hold a note in Notes.
° Tap Open in New Window.
The item opens in the center of the screen, on top of what you’re viewing, without otherwise changing your view.
Tip: You can also pinch open any of the items listed in step 1 above to open them in a new window in the center of the screen.
You can change a center window to full screen by tapping
Make it a Split View window by tapping
Or change it to a Slide Over window by tapping
You can also use the App Switcher to switch between apps in Slide Over.
(This how-to is based on my experiences and info on Apple’s support pages — where the images sometimes come from.)