How to set up registration for event with parallel sessions?
Make a node type for sessions, then add the registration field to it. You should make a small module which compares a time/date range/duration field on your session node types to ensure the user cannot register for sessions that are conflicting.

If you have multiple events on your site, consider making a node-reference field to another node type: "events". Then update your session conflict module to take events into account.

This is for Entity Registration (D7)