In accordance to the 2006 thru 2011 Nalc contract in which they revised and restored it in the new 2013 Nalc contract..

its on page 166

A. Installation heads may continue to fill authorized vacancies
first through promotion, internal reassignment and
change to lower level, transfer from other agencies, reinstatements,
etc., consistent with existing regulations and
applicable provisions of the National Agreement.

B. Installation heads will afford full consideration to all
reassignment requests from employees in other geographical
areas within the Postal Service. The requests
will be considered in the order received consistent with
the vacancies being filled and type of positions requested.
Such requests from qualified employees, consistent
with the provisions of this memorandum, will not be
unreasonably denied. Local economic and unemployment
conditions, as well as EEO factors, are valid concerns.
When hiring from entrance registers is justified
based on these local conditions, an attempt should be
made to fill vacancies from both sources. Except in the
most unusual of circumstances, if there are sufficient
qualified applicants for reassignment at least one out of
every four vacancies will be filled by granting requests
for reassignment in all offices of 100 or more man-years
if sufficient requests from qualified applicants have
been received. In offices of less than 100 man-years a
cumulative ratio of 1 out of 6 for the duration of the
National Agreement will apply.

C. MSCs will maintain a record of the requests for reassignment
received in the offices within their area of
responsibility. This record may be reviewed by the
Union on an annual basis upon request. Additionally, on
a semiannual basis local Unions may request information
necessary to determine if a 1 out of 4 ratio is being
met between reassignments and hires from the entrance
registers in all offices of 100 or more man-years.