Some prompts require a yes-or-no response. A yes/no prompt will accept either yes or y for a positive response, and no or n for a negative response. yes/no prompts are used for confirming potentially dangerous actions such as overwriting an existing file, exiting without saving case, data, mesh, etc.
Some prompts require actual Scheme boolean values (true or false). These are entered with the Scheme symbols for true and false, #t and #f.