Complex form processing steps?

I have question regarding the "Add to Shared Contact List" processing step in Eloqua forms.

When I select "Use a picklist to select the contact list", and then choose the correct form field with the expected options — what exactly do I need to do in the third setting:



“Use a lookup table”?

Could you explain how that part works and what needs to be configured?

Also, looking ahead:
If I later decide to use the option “Use the value of a form field to select the contact list”, how does that method work?
What would I need to set up in Eloqua (or behind the scenes) to make sure that approach functions correctly?