Bookable items in Eventsforce are questions which can have capacities and prices added against each alternative. Bookable items are best used when you have optional activities that attendees can select at an additional cost (example: Will you attend the gala dinner?).
Bookable item question types are limited to 'radio buttons', 'drop-down boxes', and 'multiple checkboxes'.
Adding a bookable item
Go to Website > Content > Registration Pages
Click to "Edit" the registration page which should contain this question
Hover over 'Add here' and select "Bookable item"
Search the database for an existing question or create a new one
Choose the question type
Add the answer alternatives attendees can choose from