What "identifier" should I pick?

[carcinocron]

Are there namespace rules for this kind of thing? Best practices?

[FabianLars]

In general it follows the reverse domain name notation. We don't recommend it, but it should be possible to use other formats (or just the app name) in case you don't care about app store support (they generally require this format iirc), but it's not officially supported by us...
The most important thing is that it must be unique for each app.

[JonasKruckenberg]

This would be great to add to the docs FAQ as-is. How do i bookmark something 😂

[FabianLars]

I always leave the tab open in a separate window and then close that after a month of ignoring it. 10/10 would recommend.