Related Website Sets (“RWS”) provides a framework for developers to declare relationships among sites, to enable limited cross-site cookie access for specific, user-facing purposes. This is facilitated through the use of the Storage Access API.
Browsers have proposed a variety of tracking policies and privacy models which scope access to user identity to some notion of first-party. In defining this scope, we must balance two goals: the scope should be small enough to meet the user's privacy expectations, yet large enough to provide the user's desired functionality on the site they are interacting with. Related Website Sets (RWS) is a web platform mechanism, proposed within the context of browser efforts to phase out support for third-party cookies, through which site authors of multi-domain sites may declare relationships between domains such that the browser may understand the relationships and handle cookie access accordingly. The core principle of allowing browsers to treat collections of known related sites differently from otherwise unrelated sites is grounded in ideas that had been previously discussed in the W3C (such as Affiliated Domains), the now defunct IETF DBOUND working group, and previously deployed in some browsers (such as the Disconnect.me entities list).
Docs: https://docs.google.com/document/d/1Lbvn3Wt664AhWA-UytjGEi7UcRMhrR4trUWEi2ieUkE/edit#heading=h.t7ybo54eelkd https://docs.google.com/document/d/16m5IfppdmmL-Zwk9zW8tJD4iHTVGJOLRP7g-QwBwX5c/edit?usp=sharing
Explainers: https://github.com/WICG/first-party-sets