As an example I use the iOS app when commuting and the web app at work. When I get in to work I would like to be able to open the web app and for it to be playing what was playing on my phone. When I leave the office the opposite would happen; I could pick up the same track at the same position that was playing on the web app on the iOS app.
Unfortunately not for audio files. Plex doesn’t store progress within a playlist either.
This works currently only with video files. You can stop playback on one device, then go to the next wioth a plex client. The in-progress video will be waiting for you on the home page, ready to be resumed.