A number of Nations used privateers to prey on their adversary's vessels. A private vessel might be issued a letter of marque that allowed the privateer to attack ships of a specific nation (or nations), without being accused of piracy.
During the wars between Alliance of Free Stars and Republic of Cinnabar, both belligerents used privateers. In 5147 CE, they constituted a threat to Cinnabar shipping, because the RCN was too tightly stretched to patrol against them.1:1
Several decades before 5147 CE, when Admiral Torgis was presumably a junior officer, he rescued the son of a Cassis prince from an Alliance privateer, earning him a gift of a fine scaleware service.2:14
Privateers were not the most reliable forces, and were often barely removed from pirates. Some operated with warrants or letters of marque from multiple governments, and determined who to attack based on their own self-interest rather than any loyalty.6:12