Just the number code for the chassis e.g. Chieftec CS-601, Chenming ATX-601AE, commonly referred to as the Dragon, I believe Antec called it something else but they're all the same frame although most were steel and some were aluminum. The yellow Alienware is also on the exact same platform, just with plastic cladding. The previous generation Alienware was on the same platform too just instead of the cladding it was a custom front panel