Use of Related Website Sets for defining 1st and 3rd party context

[brgoldstein]

The Chrome team is requesting feedback from the ecosystem:

Chrome's IP Protection proposal includes relying on Related Website Sets (RWS) for the purposes of determining first-party and third-party on a contextual basis. When building the Masked Domain List (MDL), RWS are taken into account, meaning that a site sharing an RWS with the Top Level Domain (TLD) will not be proxied, even if on the MDL.

Sites are encouraged to submit or update their RWS before IP Protection is launched. If a submitter is defining a same-party relationship for non-browser-storage use cases like IP address, utilizing the Service subset is recommended, where possible. This would avoid utilizing one of the limited Associated subset slots and avoid granting that domain higher storage access privileges than may be necessary (principle of least privilege).

We are interested in feedback on whether there are critical use cases for IP address data sharing among RWS candidate sites (i.e. domains you own) in a 3p context in Incognito where the Service subset may be insufficient, such as embedding many sites you own that are also browsable TLDs, which cannot be Service domains.