There are different types of pins that the user may want to leave disconnected. No Connect means that the package pin is disconnected from all internal functions and therefore do not need to be connected. Do Not Connect means that the pin is used internally; connecting to this pin in any manner will yield unpredicatable results. Unused LVCMOS, LVDS, LVPECL, HCSL and CML outputs can be disconnected, ideally with no trace length, just the footprint pad.