At any Quest Board (or by talking to The Handler) you’ll be presented with four options: Assigned, Optional, Investigations and Events. Anything that’s listed in the "Optional" menu count as optional quests. Many of these can be unlocked by talking to NPCs as you advance through the story, but some optional quests are unlocked by completing assignments. A big difference between optional quests and assignments is that the former can be repeated, while the latter cannot, making optional quests one way to repeat hunts to gather materials… albeit a less efficient way than by doing investigations. Complete any fifty optional quests - including any number of repeated optional quests - to unlock this trophy/achievement.