One of:
 <B>io
 <D>irectory
 <F>ile unformatted
 <I>nfo file
 <L>ast five messages
 <N>ew messages
 <O>ld messages
 <R>everse
 <T>extfile
 <U>serlist
 file using <X>modem
 file using <Y>modem
 file using <Z>modem