What is the difference between the -d and -i flags in the caffeinate command?

Question

The man page for the caffeinate command has the following for the two flags:

-d Create an assertion to prevent the display from sleeping.
-i Create an assertion to prevent the system from idle sleeping.

I don't understand the difference between the display sleeping and the system entering idle sleeping. I've tried googling this and found the following pages, but none of them seem to address the difference between display sleep and system idle sleep.

Difference between sleep, hibernate, etc
How can I tell the difference between display sleep vs. computer sleep on my MacBook Air (Late 2010)?
Use caffeinate to prevent sleep on lid close on battery
https://mackeeper.com/blog/mac-sleep-modes-explained

What exactly is the difference between the display sleeping and the system idle sleeping?

Answer 1

I would assume that 'display sleeping' means that the display goes black but the system is still running. e.g. checking for mail & running apps continue to run.

'system idle sleep' sounds like what happens when you choose 'sleep' from the Apple Menu. The display goes black and processing stops and the Mac is asleep.

There is (or maybe was) a feature called 'PowerNap' where some functions would still continue even when the system is/was asleep. I'm not sure how that affects system sleep.