There's also a lot of waft in part 121 about operator set minimas and such, which makes a lot of stuff happen behind closed doors. While there are factors to be applied as guided what approach minimas they use are up to the operator and get pretty low if you are using the airport as an alternate, well below what a Jepp plate suggests. It's a minefield for pilots and a lot of theories yet to be tested. In theory I could use Melbourne or Sydney as an alternate with Cat 1 only capable with as low as 450ft ceiling, allowing only 200ft margin, legal, but prudent?