It's referring to the status of the drain. An "N.O." drain remains open when it's not activated; an "N.C." drain is normally closed when it's not activated.
This allows flexibility in your filter setup. Normally open drains help prevent problems caused by freezing since the condensate doesn't build up.
For both NO and NC SMC drains the drain can be discharged manually by turning the drain cock to the "O" position.
Here's a link to SMC's NO / NC FAQ illustrations that will provide more details; click on the question if the pictures don't load right away.