-dwgunits vs insunits

Can I start off by saying, I think I fully understand the following(INSUNITS):

"Have you ever inserted a block or an entire drawing into your current drawing and noticed that it 'magically' scaled upon insertion? It either came in way too small or too big and it's driving you crazy.

"There are several different combinations of insertion unit variables that you need to be aware of in both the source (block) drawing and the target (current) drawing.

"First, in both the source (block) and target (current) drawings you'll need to look at the INSUNITS variable. This variable is stored in the drawing so it could change from drawing to drawing.

"This is the value shown below that you can access from the Format pulldown - Units.

"For example, in the source (block) drawing, if this is set at inches and that file is inserted into the target (current) drawing where it is set at feet, it will be scaled by the ratio of the two units. In this case, 1 foot/12 inches so the block is scaled at 1/12th the size when inserted.

"Likewise, if the source (block) drawing INSUNITS is set to feet and is inserted into the target (current) drawing set to inches, it will be scaled x 12 when inserted. (12 inches/foot). "Second, another option for INSUNITS is Unitless. This setting can cause headaches if you don't understand how AutoCAD treats a file with this setting.

"If either the source or target INSUNITS variable is set to Unitless, AutoCAD will look to the settings below to scale, or not scale, the drawing. (Tools -Options - User Preferences).

"The two system variables that AutoCAD will look to when handling scaling with Unitless settings, and stored in the computer registry, are INSUNITSDEFSOURCE and INSUNITSDEFTARGET.

"Example: If the source (block) drawing's INSUNITS is set to Unitless and is inserted into a drawing whose INSUNITSDEFSOURCE is set to feet, AutoCAD will treat it as feet and insert it based on that unit.

"Let's say the target (current) drawing's INSUNITS is set to feet. The source (block) is now treated as feet because INUNITSDEFSOURCE is set to feet in the target. Feet inserted into the target file feet, so there will be no scaling."

Now, my question.....

Where does -DWGUNITS fit in?

I understand that DWGUNITS is the units that are being used in the current drawing, but when you change the DWGUNITS it gives you an option to match the INSUNITS to the DWGUNITS and whether or not you want to scale upon insertion. So does this then mean if you choose "yes" on these two options that this overrides whatever INSUNITS, INSUNITSDEFSOURCE and INSUNITSDEFTARGET are set to? If these commands are all set to unitless, that should mean objects are not scaled when inserting, but when setting the DWGUNITS, there is no unitless option and so therefore if you press "yes", does it override what INSUNITS, INSUNITSDEFSOURCE and INSUNITSDEFTARGET are set to

Can someone please explain?

Thanks in advance.