In iOS 13, Slide Over goes from being a useful but annoying novelty, to being an essential utility. Instead of just allowing you to attach an application window to the side of the screen and swipe it out to view or edit quickly, Slide Over is now fully integrated.
On iOS 13, you can have multiple Slide Over panels, you can switch between them as easily as switching applications on an iPhone, you can open almost anything on a Slide Over panel, and you can easily convert a Slide Over application into one full screen application. This is how everything works.
Slide Over on iOS 13: finally useful
In the past, Slide Over proved to be quite useful. For example, you can drag the Notes application to the side of the screen and then use a gesture to slide it when you need it. It worked as an iPhone application, with an iPhone-sized window, and was fully functional.
The problem was that, if an application was in Slide Over, it could not be anywhere else. If you place Safari on Slide Over and then touch the Safari icon later to open it normally, the Slide Over window will appear. To solve this, I had to drag carefully to convert the panel into a split View panel, and then drag that panel out to appear in full screen. Absurd.
iOS 13 corrects all this. You can now have as many active application windows as you want, so you can put a single Safari tab in Slide Over and continue using other Safari windows as normal. You can easily drag a Swipe over panel to swipe it to full screen (as well as drag anything to Swipe over, even links). And you can switch between this panel nest, allowing you to keep several applications on one side of your screen, to recover them instantly.
Let's look at the three essential new features of Slide Over.
The new Slide Over application switch
The Slide Over application switch is at the heart of the new configuration in iOS 13. If you have multiple applications in Slide Over, you can switch between them, as well as change the applications on the iPhone X. See the small bar or "Handle" at the bottom of the panel? Simply swipe over to change application. A quick slide will alternate between the two most recent active applications. A longer slip will place it in the Slide Over Exposé application (or whatever it is called). Here it is in action:
When all applications are deployed in this Exposé-style view, you can slide any of them to force the application to close. And you can also …
Drag a panel Swipe over full screen
You can take any panel and drag it to the top of the screen, which will become a normal full-screen application window.  Drag almost anything to Slide Over
You can also drag anything to Slide Over. A link, a Dock application, an application in Spotlight search results. You can even drag a notification and convert it into a window. Here it is with a link:
Slide Over on iPadOS is a big problem
Slide Over is much better on iOS 13. Now, you can keep a group of mini-windows aside, ready to be dragged. That could be the Notes application, a reference web page, your password manager application, anything. I keep the Yoink shelving application there, so I can quickly save and take items from the clipboard. I also keep Slack there while I'm working, to quickly check messages when I see an alert. And speaking of messages, this is a perfect place to place your favorite messaging application.
Because you can now keep multiple applications in this space, it's like having a small iPhone built into your iPad, ready to use at any time. I use it a lot. I just wish there was a keyboard shortcut to hide and show the Slide Over shelf.