In the tradition of Earth military units through the centuries officers and crew of any post are issued a special large coin when they have been assigned to such a post. This coin, in the past, has carried many forms and at times has served as a kind of special identification token. Such coins have been collectables throughout history as well as family heirlooms, tokens of trust and friendship, etc...
Design
Solas Tempus uses such coins. Upon being formally assigned to a new posting the C.O. will issue a special coin - 6cm across and 50mm thick - with the embossed unit insignia on one side and the Solas Tempus insignia on the other.
Written in embossed lettering around the Solas Tempus insignia the phrase; "Semper protegens, solus per tempus." and the year 2360 centered between the beginning and end of the text. (Always protecting, alone through time.) The motto of the individual unit will be embossed around the unit insignia on the opposite side along with the year of issue centered between the beginning and end of the motto text. Deeply engraved along the outside edge of the coin are the words "Memor praeteritorum vivamus in hoc habere fidem futuri." (Remember the past, live in the present, have faith in the future.)
Commemorating Loss
The message around the side was not present until 2368 commemorating the events where the USS Rising Sun and USS Reed were sent back in time but the USS Rising Sun did not return. It was debated at the time whether time travel should be used to go back and rescue the crew. Fleet Admiral Poole decided against it, as they should have follow their own rules - not going back in time to change events, but to set them along the correct path.
Disposal of Old Coins
Standard procedure holds that if a person does not plan to retain their challenge coin they return it to their C.O. but it is not required. The coins themselves hold little value outside of their symbolism being made of a titanium alloy. They do, however, add a legacy feature where the names of the command crew on the day of the coins creation are etched on the inside of the metal using a subspace particle beam. It is invisible unless scanned for.
Many coins are kept by officers to denote a history of their service, operatives often keep the first coin they received as a temporal operative even if they don't care to keep the others. It is widely considered a gesture of friendship and great trust to give one of your coins to another person and family members often pass down coins to children or relatives through generations.