Aircraft approach category question

[WhySoTough]

"The criteria taken into consideration for the classification of aircraft by categories is the indicated airspeed at threshold (VAT) in landing configuration at the maximum certified landing weight i.e. VAT = 1.3 VS or VAT = 1.23 VS1G (fly-by-wire aircraft)."

I don't quiet understand this, especially the 1.23 VS1G.
Would appreciate some explanation please?

Also while on the topic, I want to understand why an a330 is a CAT C whilst an a321. Is a CAT D?

Thanks in advance guys.