From Online Manual
A form of visual verification may be present in order to confirm that a guest is actually a person and not a bot. This verification may be requested in two ways - CAPTCHA and custom questions. The CAPTCHA method displays an image containing a random collection of letters, which a guest must enter into a text field. The custom questions method displays one or more relatively simple questions, which a guest must answer correctly. A forum may be configured to use both methods at the same time.