Liberty captains were basically freelancer captains. However, a license was required to be one. A liberty captain license granted a person a permit to own a warship legally.
There was no requirement to become a liberty captain. However, a background check was conducted to those who applied to become one. This was done to filter out possible spies and criminals. Once that was done, one had to go through a 4-week course for a basic captain training.
The original intent of this program was to provide civilians a chance to become a legitimate captain.
Liberty captains were provided with support at authorized ports and planets. They were able to purchase supplies at cheaper rates. They were also paid a small wage every season (3 months). Liberty captains were generally capable of supporting their own ship and crew, so the small wage was more of a bonus to them.
"Liberty captain" program was launched by president Gawain. At that time, Andromeda cluster was a new frontier to be explored. President Gawain used liberty captains to explore the clusters throughout for very little cost to the government.
After Gawain passed away, it seemed almost as if purpose of the program was lost. Andromeda cluster was explored fully and it seemed there was no need for liberty captains. Liberty captains were left in dark with no tasks to be had.
Eventually, however, liberty captains found their places. They became privateers literally, hunting down pirates for bounties and carrying out special tasks for the riches.
When the program was launched and supported fully by president Gawain, there were more than 10,000 liberty captains in service. Their number was reduced to mere 1,000 after Andromeda cluster was fully explored.
By year of 9599, their number was less than 100.
When United Sol fell on 9670, all liberty captain licenses were revoked effectively. There were still liberty captains however, but they were classified as pirates.
However, Freedom colony and Venus still supported liberty captains and provided them with supplies at cheaper rates.
Some liberty captains were legendary.
Liberty captain "Oraekyn" was the prime example. He was known to be the best liberty captain who fought tens of thousands of opponents single-handedly. This was not exaggerated. He did fight Juron’s' fleet single-handedly during the first Sol-Andromeda war. Of course, he was unable defeat the fleets alone but he did show them his skills. Furthermore, he was able to get away, placing a big shame on Juron’s fleet and giving a dent on their moral.