Indeed. It seems like the new features are all about focusing on single workflows that involve multiple apps, rather than being anything about "multi-tasking" (trying to get multiple orthogonal things done concurrently.)
* Split Screen is obviously for things like taking Notes from a page in Safari. One input app, one output app. No more switching. (Interestingly, together with its enhancements, this makes Notes.app a real competitor to Evernote.)
* Side Frame is obviously for "watching" one app for (slow, asynchronous) updates you care about, while doing your regular stuff in the main pane. Like keeping an eye on a Slack conversation that you're not actively a part of, while working. Side Pane could also be for building "accessory" apps that give you extra controls for another app. I could imagine running your Unity game in the main pane with a Unity editor/debugger/inspector in the side pane, for instance. Instead of every Unity game having to ship those controls in a hidden way, they just run a socket server that the other app attaches to.
* Picture in Picture is just a combination of the above, specifically for video content. Listening to a presentation while taking notes on it or doing other things, looking down at it whenever it seems something interesting happened in your peripheral vision. Would be great for iTunes U, where a lecture can be in PiP and the course's iBook can be in the main view.
I will note that Side Frame greatly resembles a similar Windows 8 feature, which didn't turn out well, though. If Apple wants their version to live up to their expectations, they'll perhaps have to limit apps from having Side Frame view-controllers unless it actually makes sense to be using that app as part of a workflow. Otherwise, we'll get the W8 "Email inbox in the Side Frame" design that is just constantly distracting people, instead of encouraging them to set up filters and proper notifications.
* Split Screen is obviously for things like taking Notes from a page in Safari. One input app, one output app. No more switching. (Interestingly, together with its enhancements, this makes Notes.app a real competitor to Evernote.)
* Side Frame is obviously for "watching" one app for (slow, asynchronous) updates you care about, while doing your regular stuff in the main pane. Like keeping an eye on a Slack conversation that you're not actively a part of, while working. Side Pane could also be for building "accessory" apps that give you extra controls for another app. I could imagine running your Unity game in the main pane with a Unity editor/debugger/inspector in the side pane, for instance. Instead of every Unity game having to ship those controls in a hidden way, they just run a socket server that the other app attaches to.
* Picture in Picture is just a combination of the above, specifically for video content. Listening to a presentation while taking notes on it or doing other things, looking down at it whenever it seems something interesting happened in your peripheral vision. Would be great for iTunes U, where a lecture can be in PiP and the course's iBook can be in the main view.
I will note that Side Frame greatly resembles a similar Windows 8 feature, which didn't turn out well, though. If Apple wants their version to live up to their expectations, they'll perhaps have to limit apps from having Side Frame view-controllers unless it actually makes sense to be using that app as part of a workflow. Otherwise, we'll get the W8 "Email inbox in the Side Frame" design that is just constantly distracting people, instead of encouraging them to set up filters and proper notifications.