This has been subject of discussion previously. It's all well and good having the 'one spring fits all 'approach for the rear springs, with only variations between saloon and estate, but GM saw a necessity to fit a number of different springs relative to different models, with differing ratings applicable to the various springs.
In my own experience, fitting aftermarket springs of the wrong wire diameter, or non progressively wound springs, or simply of the incorrect free length may give a incorrect ride height.
Some owners don't mind the back end of their Omegas sitting too high, or looking like a dragster, but in my view there needs more information on the aftermarket springs being fitted to ensure they are as near to the spec of the originals. One spring fits all approach doesn't take GM's thoughts into account for the differing rear springs across the model range into consideration.
GM identified the different springs with various letters, such as LA, LK etc etc etc
Only saying....