In general, a grade of a B- is not counted towards credit for a grad student. Thus, as a courtesy, if a student receives a grad less than a B- in a grad class, I usually change their grade to a W (withdraw) so as to not impact their GPA.
Please let me know ASAP if you do not want this option i.e. you think you may get less than a B- and you prefer to receive a letter grade less than a B- rather than a W.
Jared