What is a Device Firmware Reset?

According to the iPhone service guide (any model):

"A Device Firmware Reset may resolve an issue that caused the

device to not turn on and all other attempts have failed".

Then it explains how to do a DF Reset:

Timing is critical — use a watch to time the steps below:

1. Connect the device to an AC power adapter. Do not connect the device to a computer.

2. Press and hold the following two buttons together for 15 seconds, then release the Sleep/Wake button.

iPhone 6s and earlier, iPad, and iPod touch: Sleep/Wake button and Home button

iPhone 7: Sleep/Wake button and Volume down button

3. Continue to hold the following button for 20 seconds:

iPhone 6s and earlier, iPad, and iPod touch: Home button

iPhone 7: Volume down button

4. Leave the device connected to the charger for five minutes.

5. After five minutes, press and hold the two buttons listed above (depending on device model) for 20 seconds or until you

see the Apple logo.

I suspect that a DF Reset consists of resettting the firmware settings to defaults. But this sounds to me a bit strange, my idea was that Firmware can be restored (erased and reinstalled) but not altered in any other way.

I wonder why the service guide for No Power cases, doesn't include neither a DF reset nor a DFU nor a Recovery mode restore, it just prompts you to reset the device and check if it's connected to iTunes.

Also: I guess a DFU restore includes any benefits a DF Reset may do, so it's pointless to try a DF Reset after a DFU reset, right?.