Combine Missions are an essential part of the game, the purpose of which is to reward players for teamwork.
A player can earn up to four Reward Packs during a single Mission, namely Basic, Standard, Elite, and Exclusive Packs. Reward Packs are stored in the Market > Special tab and can be opened within 72 hours from the moment when a player receives them.
After 72 hours the Reward Pack expires and a player will not be able to open it anymore. Make sure to open yours every day to maximize the rewards.
Unlike personal Quests and Tournaments, a player receives the Reward Packs not when the Mission ends but at the moment when Combine fulfills the requirements for each Pack. It means that you can open the Reward Packs when the Mission is still in progress. Often players are getting confused because when the Mission ends, a list of unlocked Reward Packs is displayed again, even when a player has already opened all Packs.
If you can't find the Reward Packs in the Market > Special tab, it means that you have already opened them earlier.