PWA Studio uses UPWARD in development. UPWARD is an acronym for Unified Progressive Web App Response Definition. An UPWARD definition file describes how a web server delivers and supports a Progressive Web Application.
UPWARD definition files provide details about server behavior using platform-independent, declarative language. This lets a Progressive Web Application run on top of an UPWARD-compliant server in any language on any tech stack because the application is only concerned about the HTTP endpoint behavior from the UPWARD server.
An UPWARD server uses a definition file to determine the appropriate process or service for a request from an application shell. It describes how the server should handle a request and build the response for it.
A PWA project can include an UPWARD definition file to specify its service dependencies.