Various printer brands feed the ribbons through their machines differently. CSO (coated side out) ribbons are made with the ink on the outside of the ribbon spool. They unroll from the bottom before making contact with the print-head and label. CSI (coated side in) ribbons, on the other hand, have the ink on the inside of the spool. They unroll from the top before making contact with the print-head and label.
Unsure which type you were using before?
Stick a piece of tape to the outside of your ribbon roll. After a few seconds, peel the tape off. If ink sticks to the tape, you were using a CSO ribbon. If no ink sticks to the tape, you were using a CSI ribbon.
Be sure to also read our article, "How to Choose the Right Type of Thermal Transfer Ribbon."