It is used a little differently from the others. In the other options, exercises puts a section or subsection heading on each section of exercises and writes the appropriate heading for the answers to that set to the answer file. In this option, exercises does neither. You must write the title of the exercise section if you want one, and you must write the title of the answer section, putting in a page break and setting the page number to 1 if that is what you want. The document ``onceexam.tex'' illustrates the use of the once option.