Well, they mean exactly what they say.
If you want a normal interactive VM, keep the default "User that powers on the VM" option. If you want a back-end service VM, change to either the SYSTEM account or a specific user/password account. Then you can change the startup/shutdown options below.
P.S. You REALLY should read the user manuals. Virtualization software is much more complex than a word processor or other application you typically run... the manuals really are necessary. And 90+% of the questions you've asked here are answered in the manuals.