What files/devices should be excluded from Time Machine backup?

I recently added a portable SanDisk Extreme drive to use for backup. I already have 2 hard drives that I rotate use of. When I added this 3rd one, in options says under General>Time Machine>Options

"Exclude from backups"

The new drive was there, but also some obscure file reference which I accidentally deleted. Was this something important, that should be excluded from every backup? I don't know if it was just for that drive, or was on all the drives. Because if I highlight each of them, it just has my new drive listed. On Apple it says TimeMachine excludes system files and apps from backup. I assume that is automatic and behind the scene.

I only connect one drive at a time.

Have I screwed something up? Thank you!