IIRC it's what type of computer you have (e.g., Windows 10, Unbuntu, MSDOS). I entered "Windows 10 Lenova Yoga".
If you're sending the ECU to them for tuning I doubt it matters. That is there (I imagine) if you're doing a remote tune yourself where they send you the file.
2006 Lotus Exige: Avery Matte Graphite wrap, partial built motor (9:1 Mahle pistons, Ferrea Competition valves, ACT Clutch, ARP head studs), REV400, Nitron SAs, BOE QS exhaust, Ultradisc rotors, Moroso Oil pan, Circuitworks oil pump, CF Stuff (Cup Roof, ReVerie diffusor, side scoops, APR air dam, craftsquare mirrors), Worksbell quick release, Odyssey PC625, Morimoto Xenon HID, BOE Rear Oil Cooler, Difflow Aeroplates, BOE Swirl Pot, DaviesCraig EWP, Reversed Heater Matrix