iOS 18: What is the difference between iCloud Calendar and Calendar showing in iOS iCloud Settings

iOS 18: There are different setting toggle options shown for two Apple iOS calendars in Settings > iCloud > Saved to iCloud.

(i) iCloud Calendar

(ii) Calendar.

Since both the calendars are showing in iCloud settings, it is evident that the data of both calendars will get stored in iCloud. In this condition, the reason for showing two calendars (both from Apple) showing in iCloud settings is not understood. Further, the difference of these two calendars is not showing in the Calendar app of iOS.

What is the reason for having two different calendars and the difference between these two calendars: iCloud Calendar and Calendar showing on iOS iCloud > Saved to iCloud settings.

Also, this difference in the two calendars, if any, must be explained and visible in the Calendar app of iOS.