bios - What exactly is "UEFI with CSM" boot mode?

Under this question, I was told that "UEFI with CSM" is essentially BIOS mode. It made me quite confused because what I've learnt over the years is far from that.

As per my observation, I think CSM is a special mechanism that allows previous "not canonical" UEFI OS to boot.

Here's my observation:

1. The full name for UEFI with CSM is

   
   
   

   
   
   

   UEFI with Compatibility Support Module

   
   

   
   
   

   which as its name suggests, is a "supprt module" for UEFI.




2. I have three partitions on the primary drive in my computer. The drive is formatted to GPT and the three partitions are one ESP (EFI System Partition) and two Primary (Windows 10/Windows 7 each one). There are three options for "Boot Mode" in the motherboard settings, "Legacy", "UEFI" and "UEFI with CSM".

   
   
   

   If it's set to "Legacy", the computer tells me "No operating system found". That's correct. If it's set to "UEFI with CSM", both Windows boots normally. If it's set to "UEFI", then only Windows 10 can boot. From the safe boot log I can see Windows 7 gets stuck at classpnp.sys.

   
   
   

   I tried removing \Windows\System32\winload.exe from both volumes (I did that in a Windows To Go environment), and nothing went wrong, so I restored the file.




3. I plugged in another drive in MBR, containint two partitions. One primary active with FAT32 and another primary with NTFS, with another Windows 7 installed. It seems I however just can't make it boot if the MB settings is set to either "UEFI" or "UEFI with CSM", but it boots perfectly when set to "Legacy". If I edit the BCD in the EFI partition on the primary disk to add an entry for this experimental Windows 7 installation, it boots under "UEFI with CSM", but still gets stuck at classpnp.sys in "UEFI".





4. I have another computer with a MSI B85 motherboard that has a switch named "CSM". If it's enabled then there are two options available in "Boot Mode", "UEFI" and "Legacy". If it's disabled then Boot Mode is locked to UEFI. In that case "UEFI" mode with CSM enabled allows Windows 7 to boot, but it won't boot with CSM disabled.

The above experiments made me believe "UEFI with CSM" is a special mode of EFI, so why am I still told it's BIOS?