Reserve Cases are a feature within the Black Market of Call of Duty: Black Ops 4. They are similar to Reserve Crates, in the fact that they award the player with a Reserve exclusive item, however, they are only earnable by playing about an hour of Multiplayer, Blackout or Zombies, and only give 1 Reserve item. Contracts were added in later, giving optional challenges which reward players with up to two free cases along with experience points for the respective mode.
They are also earnable in Black Operations, which is unlocked after the current Operation has concluded or the player has completed all available tiers.