In reading the use cases as well as the definition of subsets I have a use case that does not seem to be addressed that also presents a need for a clarifying question on the first bullet of abuse mitigation - "Mutual exclusivity to ensure a domain isn't part of multiple First-Party Sets" .

Does FPS cover the use case of websites that utilize a 3P vendor for booking/purchasing? The easiest example is a small hotel that utilizes one of the large booking engine companies to handle date availability and booking confirmation.

User first goes to hotelpropertsite.com and navigates to search for their dates of stay. Once the User submits the dates of stay, they are taken to the bookingengine.com url to select the available room types and continue their purchase. In this case the hotelpropertsite.com is the "set Primary" and bookingengine.com is the "set member". Because bookingengine.com has the same branding and structure but not common ownership, it would appear to meet the definition of the "associated" sub-type. This is where the Mutual exclusivity clarification is needed. Is ONLY the Primary in the set subject to this abuse mitigation measure or does it apply to the Member as well? In the case of bookingengine.com, this domain could be an associated set member to thousands of hotels that utilize the booking engine for availability and booking.

This same use case could be a personal website that links to etsy.com to sell goods or a high school bookstore website that links to shopify.com for their apparel. In these cases, the association is obvious and the ownership is clearly different. But if the mutual exclusivity applies to any domain in a set, then these larger 3P vendors would appear to go against this abuse mitigation measure.