Concurrent sessions are sessions scheduled in the same time slot, so attendees pick one and organizers size rooms around the likely split.
Every concurrent slot forces a choice, and every choice means somebody misses something. The craft is in which sessions you put against each other. Two sessions aimed at the same audience in the same slot is a self-inflicted wound; pairing a technical deep dive against a business-track session lets most people attend what they came for.
Session-level registration is the only real forecast you get. Without it you are sizing rooms on intuition, and the session you underestimated is the one with people standing in the corridor. With it you can still move rooms while there is time.
Moving a few hundred people between rooms takes longer than the ten minutes most agendas allow, especially when the rooms are on different floors. Sessions that start late then run late, and by mid-afternoon the whole program has drifted. Build the transition time against the real walking distance.
Each session carries its own capacity and registration, so you can see the split before the day rather than discovering it at the door. Attendees register for sessions at checkout or afterward, and session attendance is reported separately from registration. See event agenda builder.