@AngelinaIgnatova, oh, maybe I misunderstood your post on RFC-25 where your screenshots show text about “fields associated with an app”. That is what I mean with “locked custom fields”. Please correct me if I have the terminology wrong here.
Anyway, if I understand you correctly then these fields associated with an app are only available for Forge apps, not Connect apps, right? Your links point to the Forge docu, but I don’t see similar pages about such “locked” fields in the Connect documentation 
Since our app is a Connect app, we would need it for Connect.
Thanks for any clarification.