In the ancient days, the letter of marque was a licence granted to an individual or group of individuals by the government to authorize them to either capture, or attack vessels which were perceived to be carrying enemies. Such vessels would be presented before courts for censuring and or sale. Some of the acts that one was allowed to commit by virtue of possessing the letter of marque would have otherwise been considered to be piracy. Therefore, having the letter was perceived to be an ethical way of blending both profit and patriotism.
In specific regions such as France, the letter defined the type of vessel under use, and that has been tasked with receiving the cargo when need be. It allowed the bearers to go beyond international borders in a bid to carry out retaliation. Read about Viggo’s plot in the Alatriste movie of 2006.