Short answer: Open the Dashboard and tap Injections, then Site Rotation. Pick your Default Rotation Sites and set the Rotation Window. When you log a dose, the app then suggests the site that has rested longest and warns you if you pick one used too recently.
In the log sheet the Injection Site row shows the suggested site and when it was last used, for example Last used: 9 days ago. A line under it reads Auto-rotation on · Tap to manage. If you have not picked sites yet it reads No rotation set up. Tap to auto-pick sites!
Tap Choose Site to open the body map. Sites you used within your rotation window are marked. You can also tap Add Custom Site for a spot that is not on the map, such as "Calf - Left".
If you pick a site used within the window, you get Site Used Recently. If one site makes up more than half of your last 10 injections, you get Site Overuse Detected. Both are only warnings. Tap OK and you can still log there.
Under ROTATION SCOPE you choose whose history decides the next site. Per medication (the default) means each compound keeps its own place in the rotation. Whole body means all compounds share one rotation. Either way, the warning about a recently used site checks every medication, because any injection in that spot counts.
On the Injection Sites page, tap History to see Last Site, Most Used, Total Logs, injections per medication and recent activity. Skipped doses are not counted as site use. Guides on the same page has technique notes for IM and SubQ sites.
See also: Why injection site rotation matters