Seagate Exos 18 vs Exos 20 - what's the difference?

[Quartz]

I was looking at HDDs and saw that the Exos 18 TB drive is available in Exos 18 and Exos 20 variants, but was unable to discern the difference. What is the difference? I'm assuming I've missed something obvious.

 

[Quartz]

So the 18 TB Exos 20 drive is possibly a failed 20 TB Exos 20 drive. Or just one that's been cut down. Thanks.