The CNC control is the machine's brain — the interface and software that runs the program (Fanuc, Siemens, Mazatrol, Haas, Heidenhain, and others). Control generation is one of the biggest swing factors in used value: a current, still-supported control widens the buyer pool, while an obsolete or orphaned control shrinks it and drags the whole machine's price down.
How this affects buying and selling: read The Gauge, or value any machine free at The Machine Blue Book.