The first time each day that you complete an Instanced Campaign Mission or Operation on hardcore difficulty, you receive a Campaign Token. They can be exchanged for Rare Weapons, Uncommon modules or special items in the following vendors:
- Corporal Belle: Special items
- Lt. Amanda Bloom: Mk.3 Module Crate
- Warrant Officer Liu: Max level Rare Weapons and Equipment
Campaign Tokens were migrated in full from Update 1.3 to Update 1.6, but from 1.6 on, you can only have up to 20 Campaign Tokens at any given time. If you had more than 20 before the migration, you won't be able to earn more until you're bellow the 20 tokens mark[1].