Difference Of Mail In “iCloud” Vs. “iCloud Drive?”

I’m running MacOS 10.14.2 on my MacBook.
1. I set up iCloud, and under System Preferences –> iCloud I put a checkmark next to “Mail”.
2. I also put a checkmark next to “iCloud Drive”, then I click the “Options…” button, and in the new window I put a checkmark next to “Mail.app”
I don’t understand what’s the difference between checkmarking “Mail” in #1 vs. “Mail.app” in #2. What will happen if only #1 is checkmarked? What will happen if only #2 is checkmarked? What will happen if both #1 and #2 are checkmarked?
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Tom