How to add a new tab in #formTab?

Check the ARAS CUI document for adding the tabs next to CAD Document tab

Thanks, so I found this doc explaining how to add a tab (page 27), however it isn't really clear about how to set the content of the tab. In the example, a new "Tab Element Control" is created and it simply says to set "Additional data" to "{"relTypeId": "159C6D88795B4A86864420863466F728"}" which is the RelationshipType of the Part BOM. I tried to replace this with a "formId" parameter but obviously that didn't work. Where is the syntax and possible parameter of that "Additionnal Data" property described?

More tabs here refers to relationship attached to the CAD Documents item type. And relTypeId is the ID of the relationship 

So we're not free to decide what could be displayed in these tabs, it can only be relationship?

What I'm trying to achieve is to have two views for a CAD Document, a "Simplified" and an "Advanced" one. I created two forms and I would like the simplified one to be in the first tab, and the advanced one to be in the second tab. Those two forms are only about CAD Document, not other itemtype. The first tab is already displaying a form (the default one) so it should be possible to set the second tab to also do this, no?

So I found out that it is possible to create a cui_Control of type "Form Control" whose Parent property can be set to the "Tab Element Control", and doing that I'm able to have a form displayed in the second tab. However, it still is the default Form for the CAD Document, not the second one (so I now have two tabs with the same content). I guess the "Additional Data" property of the "Form Control" can be used to specify with form to display, but I didn't find the name of the parameter to set in that JSON (I tried, id, itemTypeId, viewId ...). Isn't there a specification somewhere about this Additional Data syntax?

So I found out that it is possible to create a cui_Control of type "Form Control" whose Parent property can be set to the "Tab Element Control", and doing that I'm able to have a form displayed in the second tab. However, it still is the default Form for the CAD Document, not the second one (so I now have two tabs with the same content). I guess the "Additional Data" property of the "Form Control" can be used to specify with form to display, but I didn't find the name of the parameter to set in that JSON (I tried, id, itemTypeId, viewId ...). Isn't there a specification somewhere about this Additional Data syntax?

So, for future readers, as was not able to find the correct syntax for Additional Data, so I just added a method when the form onload which is looking at the second tab and replacing the content of the iframe. Dirty but I didn' t find any other way.

I also don't know if there is a possibility to use Additional Data.
You can use a workaround: Just add an Null-Relationship. And configure your desired form as a relationship view. Then select 'Hide in all' on the RelationshipType, so it will not be shown in Relationship-Tabbar.
Now you can use the relationship id to configure additional data in your new Tab.