Clarify differences between Stanton 980/ 981 .

I have a Stanton 980 HZS with the D98 stylus which has never been used. I mounted it tonight and am intrigued by the sound. What EXACTLY is the difference between the 980 and the 981? About the HZS and the LZS, I have the impression that the difference lies in the resistance(Z) that they like to be loaded at. Do I have the right impression or is there some other reason for the HZS/LZS designations?

Thanks in advance for the replies.