This refillable cartridge contains [strong base] anion exchange resin.  Captures anions and releases hydroxide (OH^-) ions.  Used in separate bed deionization systems.  

This anion resin will change from blue to tan. When you see roughly half of the cartridge change over to tan, you'll know its time to replace. See anion refill resin here: 1 refill or for multiple cartridges, see bulk refills here