Type of eligible partners in condition contract type definition

I'm going through the configuration of a condition contract type, and there's a field whose description is Type of eligible partner. In that field, you have the following options: 1. Customer, 2. No eligible partner, 3. Plant, 4. Reference condition contracts and 5. Supplier.

I'd like to know what this field controls. Also, what's the impact of, for example, choosing the option No eligible partner?