. . Encounter Codenames are dropped from a specific ship in any given Encounter, typically one that's more dangerous than the others.
The specific ship has a low spawn rate, and once it is taken down a set amount of time must pass before it may spawn again.
The numbers between brackets indicate the spawn rate of the ship dropping the Codename.
To find the Encounter area on your map, click the second tab (Political Map) and search for small, perfectly round circles; if none appear, check for them among friendlies and hostiles tabs. You'll know you're in the right place when you start hearing Danger music.