SSC ask for the preferred post circle from the selected candidates. See details here

The candidates will be assigned to that post circle where their preference and category vacancy and number of vacancy in that circle match. The candidates need to provide their preference to get an appointment in their preferred location. In absence of the preference, the Department of Post will send the candidates to the available Postal Circle.
SSC also provided the number of vacancies available under each category in all the 17 posts circle. The candidates are requested to provide their order of preference for the Postal Circle.

See here: SSC CGL exam dates

A candidate who does not provide the order of preference or provide a limited preference will be in a disadvantageous position. The candidates who do not provide their preference and are not allocated to their given limited choice of preference shall be allocated circle in alphabetical order of the name of Postal Circle subject to the availability of the vacancy in the relevant category. SSC CGL exam is the preferred choice of the graduate candidates who want to serve under the Central Government.

The options provided by the candidates will be final and no change will be allowed. The candidates need to provide their preference in the prescribed format attached with the SSC official Notice. Please see, download and send the order of preference in the prescribed format by June 15.

See here: SSC Official Notice