Medela's CSF Bags are used to collect, store and freeze expressed breast milk. The bags are presterilized for maximum hygiene and they attach directly to any Medela breastshield so you can pump right into the bag. No need for separate collection and storage containers. A built-in pouring spout can be torn open when it's time to empty the bag. The area for labeling each bag is outside the milk chamber to protect the purity of the milk.