Whether you do the sessions in the web app, mobile apps or in the desktop app, everything is synchronized so all data is stored in the timesheets and reports in the web app. All you need to do is hit play! All sessions will sync regardless of whether you assign a client and session label, one or the other or neither.
If you intend to use clients you can create them in the client tab in the web app or you can create them on the fly in the timer and add hourly rates later in the client tab. If you don't require clients or would like to turn revenue off you can do so in preferences.
For each session completed the client and/or session label (if it is present), session number and session length is saved in the web app. If multiple entries on one day have the same client and session label, they will be totaled together, showing the total number of sessions and total duration. When you hover over an entry you will be able to see how long each individual session was and what time it started.