The wording is somewhat ambiguous. Its a publishing app for Desktops (Windows/Linux/Mac/BSD). Similar use cases as Adobe InDesign.

UP requires server-side support. Signal has ignored the request to implement it numerous times. So users need to host a proxy that uses Signal's websocket notification protocol to connect with Signal and distributes received notifications over UP. This moves the battery-draining websocket from mobile devices to some net-powered server.