By default, they appear to both be assigned to “Switch applications”. They can be re-assigned using the keyboard preferences:
- Open the menu in the top-right-hand corner of your main screen.
Click on the “Settings button”, i.e. the left-most button here:
Choose “Devices” in the left-hand column:
This will lead you to a list of supported keyboard shortcuts, with their assigned keys; you can click on any entry to change it:
The “Switch applications” entry is somewhat strange: it only shows SuperTab, but it is also assigned to AltTab and will be disabled if you re-assign the latter. However it can then be re-assigned to whatever you want. As you can see, I use SuperTab to switch applications, AltTab to switch windows, and CtrlSuperTab to switch windows inside an application.
You can assign nearly any key to any of the supported shortcuts, but you can’t add new shortcuts.
On my system, running GNOME 3.26, “Switch windows“ only shows windows on the current workspace:
whereas “Switch applications” shows all applications across all workspaces:
Note that GNOME Tweaks has an “Alternatetab” extension which can also be used to adjust the behaviour of the window switcher.