Hi ​@annfoss 

The relation diagram shows the relations set up between tables. It does not just appear when you make a lookup.

If you remove the default join, it will not show in the relation diagram anymore.

So you always make the relations before dragging in the lookup field and then use the option to use the default relations in your joins.
I show it in this guide.

Hi ​@annfoss does Thomas’ answer above answer your question? If so please help us by marking the best answer above. Please let us know if you have any follow up questions

Thank you for the tutorial, it was helpful. I am still wondering if it is possible to make a relation between tables across different data areas (e.g., DSA and MDW) ? There will not automatically be created a relation between those? 

Hi,

Data Areas are more like functional process areas (staging, combining, final MDW, etc.). You cannot relate across Data Areas. You can, however, use schemas inside a Data Area to separate things. Inside a Data Area you can relate across schemas.

Hi ​@rory.smith 

I see, thank you for the clarification!