Special Operations
This is where you will find missions that test your abilities to the limit. These will take considerably longer to complete than normal Co-op missions, and require all your skill, but you'll earn increased rewards and you can earn special achievements. If you're up to the challenge, that is...
In order to access Special Operations, you will need to reach rank 25 first. You will then start to receive special Access Tokens every day, which are required to enter the missions. You can also buy more of the Access Tokens if you run out of them.
Remember to equip the best gear you own, as Special Operations can get really tough. Attempting to take them on without preparation and a well coordinated team will almost certainly result in failure. Special Operations are all about team work!