The U.S. Office of Personnel Management maintains a list of Frequently Asked Questions regarding Veterans Services. You can find additional information for Veterans here:
https://www.opm.gov/policy-data-oversight/veterans-services/veterans-services-faqs/

Foreign Service candidates who can document creditable veterans’ service by submitting form DD-214 or other certification will be eligible to receive additional points on the Hiring Register: 0.175 for a five-point standing and 0.35 for a 10-point standing. In all cases points are awarded once a candidate has passed the Foreign Service Officer or Specialist  Assessment. Instructions on how to claim these points are provided at that time.  Qualifying, preference-eligible veterans may be appointed from the rank-ordered register at any time before they turn 65. If appointed between the ages of 60 to 65, a qualifying, preference-eligible veteran may serve five years prior to being mandatorily retired.

There are a good number of Foreign Service employees in the Reserves or National Guard, and military leave is granted so that Reservists and members of the National Guard can fulfill their obligations. For detailed information on military leave and how it works, please click here and select 3 FAM 3440.