Could someone clear up the new six person rule please?

[notevenat20]

I thought the new rule was simple but a lot of MN posters are making suggestions like this:

If you have a children’s party for 6 children in the garden and you are inside, that is legal.

Or

If you have your children asleep upstairs and there are 6 adults downstairs that is legal too.

Does anyone knowledgeable know the answer?

So what if 6 people were at your place and there was a knock at the door from great uncle wilfred all the way from timbuktoo , would you have to evict one of your family or send poor old Wilfed back on his way.

The Great Uncle should really know the law and if he doesn't, it will be a long conversation out in the street explaining this thing called covid.