An employee being laid off may displace another employee who has the least seniority points: in the same class or job series as the employee being laid off; or in any other class in which the laid-off employee previously held satisfactory nonprobationary status within the 36 months immediately prior to the effective date of the layoff. Subsection (a) of this section shall apply: first to the employee’s current appointing authority regardless of geographical area; if the provisions in paragraph (1) of this subsection are not available, to the employee’s current principal unit; or a secretary or head of a State principal unit may limit the displacement within the principal unit to one or more of the established geographical areas as prescribed by the Secretary.