The Proficient Service Medallion was a medal awarded for proficient service by an officer in an service organization. Such a medallion was known to be issued by both Starfleet and the Cardassian military.
Sometime during his Starfleet service in the mid-23rd century, Christopher Pike was awarded this medal. It, along with other awards, was listed in the commendations section of Pike's personnel file. (DIS: "Brother")
During her time on the USS Enterprise, Una Chin-Riley was awarded this medal twice. These awards, along with other awards, were listed in her personnel file, in the section "USS Enterprise - NCC-1701 Medals and Decorations". (SNW: "Ad Astra per Aspera")
Gul Darhe'el received the Proficient Service Medallion prior to 2357. (DS9: "Duet")