First welcome to the user group! It’s very nice to see new members post their first questions. But I’ll just like to give you a little advise. Don’t use too general subjects (i.e. not just Navision) and also not just as a greating (like Hi). Instead write what your post is about. You can see your two posts, I have here changed the subject to reflect more what you have asked. You should make a post in the Introduction forum, so that we know a little more about you and your background.
To your question. External Document No. is required in some tables and not required in others. So it would help if you give a little more details about your issues. It’s better to post to many details than too few, if you like to receive a prompt and correct answer.