This is a screenshot of the twrp screen after you choose "Backup":
I have the following questions:
- What is the difference between the options "Boot, System and Data" on one hand, and "System Image" on the other?
- Is the "Data" option for data of system apps and user apps, or only system apps?
- I want to take a backup so that I can get a file that I can flash with Odin (that is, without requiring twrp). Which option should I use?
- What is "Vendor Image", and how does it differ from "System Image"?
- What is the difference between "Vendor" and "Vendor Image"?

Imageisddbackup i.e. the exact copy of whole partition (to retain Verified Boot), the other one istarbackup i.e. the contents of filesystem. I don't think restoring TWRP backup through odin would be possible but I never used Samsung device. May be the Images can be restored. All of the remaining questions are answered once you know what these partitions contain. – Irfan Latif Aug 10 '19 at 16:46