Not only are they made up and worthless, they are meaningless too!
In theory at the instant of start-up you draw infinite current. However, that is limited by your speedo, which mostly cannot deliver above 500A instantaneous for a good race speedo. So if I had a 6000mAh cell rated at 90C it could apparently deliver 540A, only 500 of which I could use!
The reality is that a good race speedo can only deliver 120A-150A continuous current (and lets not get started on the speedo ratings - some about as useful as the cell ratings. Bear with me!) so a 90C is pretty much useless as the speedo can't deliver the potential of the cell.
As well as ignoring the cell ratings, take the speedo ratings with a pinch of salt too. The cell requirements are that you have enough capacity to use about half of it in a modified race, and about 1/3 of it in stock. That way you will have the ingredient that really does make a difference to your lap times - voltage. Buy a cell with insufficient capacity so the voltage drops a lot and you will always be slower than someone with a larger capacity cell. HTH
