This feature adds the 'priority' request header for all HTTP requests with the priority information for the request at the time that it was sent. RFC 9218 (Extensible Prioritization Scheme for HTTP) defines a 'priority' HTTP request header to use for signaling request priority to origins (and intermediaries). It also defines negotiation processes and protocol-level frames for HTTP/2 and HTTP/3 to carry the same priority information. The header can only signal the initial priority for a resource when it was first requested while the frame-based mechanisms allow for modifying the priority after the fact. The header can operate end-to-end to the origin servers (and provide a mechanism for the origin to override the priority if recognized by intermediaries) while the frames are limited to operating on a link level. This feature is specifically for supporting the header-based prioritization scheme.
Chrome currently sends priority frames for HTTP/3 but does not send the HTTP header (and does not prevent application code from writing the header). This will bring Chrome in-line with Safari and Firefox which already send the request header.