A temporary appointment shall be an appointment which may be terminated at any time. Temporary appointments ordinarily shall be given only when service is to be part-time, consistent with section 335.11 of this Part, voluntary, or anticipated to be for a period of one year or less, or when an employee's initial appointment in the university is made to a position vacated by a professional employee who is serving a probationary appointment pursuant to sections 335.9 and 335.15 of this Part. A temporary appointment is also appropriate whenever a position has been vacated by an employee on approved leave.