Thanks Erel. This is my current understanding, with a couple of
follow-up question in underlined italic.
Impressions
The number of times your app showed up on the App Store. (So this one seem clear. Let's assume that an impression is always an impression, regardless of how they ended up there, to keep things simple. And let's consider "showing app in list of apps" = "showing app page", which really isn't the same thing, but whatever.)
App Units
Each app purchase on a device with iOS 8 or tvOS 9, or later. How does this differ from Sales below?
Sales
I thought this one said how many apps you sold for the period. (In my report it says zero, which is correct as my app doesn't cost anything to install. It also doesn't have any in-app purchases etc.)
Sessions
The number of times the app has been used for at lesat two seconds on a device. This one require a follow-up. Do we think "use" mean "actively use in the foreground"? I mean, I have geofence things going on, so the app wakes up now and then and does its thing in the background, but surely that can't be considered "used"? Or can it?
Crashes
How many crash reports Apple has received from users that volunteered them. (Which surely is less than 100 percent of all crashes that happened, because not all allow sending them in.)
Am I correct that none of these numbers can be parsed to understand "this is how many instances of your app is actually installed for the period"? The closest we got is the sessions, which doesn't tell how many users, just how many sessions there's been? Like, if I had ONE install in total worldwide, and that user used the app for 2 seconds 100 times, that number would be 100.
I also notice that not a lot of people answered this thread, so I'm guessing I'm not the only one having problems understanding the terms. (Or perhaps it's so obvious to everybody that they can't bother explaining the simplest basics.
)