Unsure about the debug option, it is possible but I'm confident I added it as the xenforo addon as I thought you had to select one but after just trying again, it allows you to leave it blank.
Aha. That does mean you have debug mode enabled. That’s ok as long as it’s locked down to only your IP but also you need to be aware of the consequences. Enabling debug mode allows you to set an add on for various things and it makes things like the master style and language available. Any custom changes you make which are assigned to any add on will be wiped out when that add on (or XF itself) is upgraded. Similarly anything in the master style or language will get wiped out too.
Always leave the add on field blank and always make sure any changes you make are made in a non master style or language.
Debug is locked to my IP via the config and I only enable it when I want to trace any performance issues, but clearly remembering to turn it back off is not my strong point!