When creating or modifying your seating plan, sections that have seat numbers allocated will be allocated/reserved seating. It is useful to name this section (for example) 'Front Stalls - Reserved' and this description along with the seat numbers will display on the customer's tickets.
For a section to be offered as general admission, simply remove the seat numbers. A customer will be able to book this section but will not be offered the option to select specific seats. It is useful to name this section (for example) 'Rear Stalls - Unreserved' which is what will display on the customer's tickets.