Although I have a hundred things to do around the apartment, I decided to prioritize by checking my BIOS. I would like to know if my settings are 100% UEFI or whether the machine is actually booting Legacy without telling me.

Start - I have a Dell Optiplex 7010 Desktop with 16GB RAM dual-booting Win 7 Pro SP1 64-bit and Win 10 Pro 64-bit. (The Win 10 side was originally 8, then 8.1, then 10. The 10 had lots of Event Viewer errors, so I "clean" reinstalled the 10 with a USB drive. On both the 7 and 10 sides, my sfc /scannows now run clean.)

Here's what the BIOS says:

General - System Information: The BIOS is A20 (the latest).
General - Boot Sequence: Boot Sequence: Windows Boot Manager is checked, and it says "Windows Boot Manager".
Boot List Option: UEFI is selected. If I hit "View" on the right, nothing happens.
General - Advanced Boot Options: Enable Legacy Option ROMs is checked. (I wonder what would happen if I UNchecked this.)
System Configuration - SATA Operation: AHCI is checked.
Secure Boot - Secure Boot Enable: Secure Boot is DISabled. (I have NO desire to change this.)
Virtualization Support - Virtualization: Enable Intel Virtualization Technology is checked.
Virtualization Support - VT for Direct I/O: Enable VT for Direct I/O is checked.
Virtualization Support - Trusted Execution: Trusted Execution is NOT checked.

So - what am I? 100% UEFI or some sloppy Legacy mixed in?

What happens if I UNcheck General - Advanced Boot Options: Enable Legacy Option ROMs ?

What difference would unchecking that make anyway?

Why doesn't Dell publish something that actually explains all of this confusion? At least in Tagalog, which some unemployed American could translate into English.