I accidently hit a star to set (change) the default app under the first setting in System / Applications/ System 4.2.7

I hadn't noticed what the previous Default App was.

Is there a way to find out what it was or is there a standard setting that it would be set for.

You could review your database backup if you have one. If not, do you remember what page was loaded first when you opened your community? The forum index page? A IP.Pages page?

The 'standard setting' is whatever you chose as the default when you installed the software originally (if starting as v4.x) or upgraded from earlier - it defaults in the choice to Forums, so if you didn't change it at install then that would be the 'standard setting'

Any way to find out what it was? Yep, restore a prior backup to a test database and peak inside somewhere to find which one was set.

My personal suggestion - try each of them until the 'home page' looks like it was before you hit the change.

So it just controls what the home page is?

Okay, than it was forums.

Thanks

But what about the settings under System as shown below. The one that it currently shows is Content Discovery. That's the one I accidently hit. I didn't notice if it was already set for that one or one of those below it.

The default module for the system application isn't super important. It would only affect what page is loaded by default visiting /index.php?app=core essentially.