MIGX(db) Configuration: Multiple Formtabs purpose and usage

Can someone give me an example for the purpose and usage of "Multiple Formtabs" in a MIGX-configuration?

you can create different formtab-sets by creating some different configurations with different formtabs in it.

If you select this configurations in the multiple formtabs listbox, you will get a new listbox in your form-window, which switches the formtabs between the different configurations as soon as you select another item on this listbox.

This is usefull for example, if you have different layouts for each record and you want also to have different input-fields depending on the selected layout.

That's what I thought. I just can't get it to work. I tried to do it like this:

I created 2 configurations with different input-fields.

I created a third configuration with those 2 configurations choosen in the listbox and attached this third configuration as a MIGX-TV to a template. Look's like this is not the way to do it.

Ok, I think I tried everything but I just can't get it to work.

I know how to setup varying layout-boxes as described here http://rtfm.modx.com/display/ADDON/MIGX.Varying+layout-boxes directly in a MIGX-TV or with multiple MIGX-configurations.

But what I'm looking for are varying layout-boxes for MIGXdb - so for a custom database table/custom object and not for JSON.

Did anybody ever did something like this with MIGXdb an can point me in the right direction?

Thanks a lot!