These are different fare classes within Business. So you have J, C, Z, I, D etc. It varies by airline so there's no fixed order. Different classes have different restrictions (max/ min stay, advance purchase, stopovers, change fees) and cost different amounts.

On EK, the fare bucket for full fare Business Class is J, while that for upgrades and redemptions is now D. This doesn't really make a difference to the novice traveller, but when you're looking for an award, it's good to know which fare bucket you're looking for.

Similarly, P = Suites, F = First, A = discounted first, Y = full fare economy, B,M,U,H,K,V etc etc are various fare buckets in Economy.