For the benefit of the Europeans - that's 600 FEET RVR, not metres......
SARBlade, what is the reason for using an alternative Cat 1 procedure rather than straight Cat II?
212man, Not sure what you mean by alternative, but Cat I,
AFAIK, is still a 200' DH with 2400'RVR, while a Cat II is 100' DH with 1200'RVR. The Cat II still offers a higher success rate of a landing in low weather than does a Cat I ILS. Most of the world offers the Cat I approaches, it being the standard. I would assume that the company did not have enough money to spend to have a Cat II assessment done but rather got an Op Spec to cover the lower DH. Visibility would still remain at 2400'. that's just a guess mind you, all I can input into why your aforementioned company had a DH 100' on a Cat I approach.
As for Cat III, I am not bothering to look up the requirements for it nor the minima, let's just say its low and usually tied to automatic landing systems, and very low visual range.