Registration to break out sessions
1. Set up ticket types
On Event > Setup > Ticket Types, edit each ticket type that allows access to break out rooms. While editing the ticket type, click More Settings +
At the "Max sessions to be selected" field, enter a number. This number defines how many sessions (break out sessions) can an attendee select. So if you want attendees to select up to 1 break out session, enter 1 at the "Max sessions to be selected" field.
The "Min sessions to be selected field" is the number of sessions that the attendee is required to select. So if you want every attendee to select at least 1 break out session, enter 1 at the "Min sessions to be selected field".
2. Set up Sessions
On Event > Setup > Sessions click New Session to create a new session (break out session).
For an attendee that registers to a specific ticket type (for example "General Admission") to be able to select this break out session, connect the ticket type with this session by checking the check box as shown below.
Limited break out session capacity
If the capacity of a session is limited, click More Settings+ and enter the capacity at the "Maximum Capacity" field.
Example registration form
If you connect ticket type(s) with the corresponding session(s), the list of the corresponding sessions will be displayed to attendee during registration, and he will be able to select session(s).
Here is an example registration form, where a session has reached capacity and is marked as fully booked:

To include selected sessions (break out sessions) to the registration confirmation, add {{TicketSessionsNamesSelected}} to the registration confirmation email.
Hint: the Clone Session and Clone Ticket Type functions can help you create sessions and ticket types with similar properties fast.
Hint: you can also connect/disconnect Ticket Types with Sessions from the Edit Ticket Type window. Once you have created the sessions, click the View Sessions button