Having stated that the Bundle Identifier is 'unique' - and certainly that is what you hope is the case - in practice the compile utility simply takes the name you enter in the App name: field and creates an ID of the form com.bbcbasic.appname. So if for example you enter as the name of your app 'My Mac App' the bundle ID will be com.bbcbasic.mymacapp which of course is not guaranteed to be unique.
I could probably have made a better effort to make it unique, for example by incorporating your User Name in the ID, but I took the easy option! So you would be advised to give your app a name which is not too generic, to reduce the likelihood of a collision with a different app created in BBC BASIC.