NFS clients can maintain use of a file over a halt or reboot (although experiencing a failure to respond during that time), but CIFS clients cannot do so safely. Therefore CIFS clients should -if possible- be warned to close their open files. If you did not use the -t option to specify a maximum delay and there are CIFS clients with open files, the reboot command displays the number of CIFS users and the number of open CIFS files. Then it prompts you for the number of minutes to delay. CIFS files that are still open at the time the filer halts will lose writes that had been cached but not written.
reboot logs a message in the /etc/messages file (see messages(5)) file to indicate that the filer was rebooted on purpose.