DSS Key - similar to line key but a lot less useful except it allows you to see when the DSS key extension number is currently in use by its owner.
I know it's been a few weeks, but I just felt like I ought to chime in on this one line. I agree with everything else you said, but DSS keys are arguable vastly more useful, not less.
DSS keys allow you to pick up the line. Just like a multi-call, except you have to tap the button. It supports Ring or No-Ring, but no other ring options. If that was all it did, I'd agree with you. However, it doesn't just do that plus 'in use.'
The light can indicate idle (off), busy (on), ringing (blink), forward or DND (fast blink)
When idle, it acts as a speed dial key. This alone makes it more useful than a multicall on sets with fewer keys like the 5320.
If you're on a call, it acts as a 1-touch transfer key. Using DSS makes it a supervised transfer, or you can use a 'Secretarial Key' which is identical but is a 1-touch blind transfer.