This from the bbc seems clearer:
You can only have one bubble per household - and you can't switch them around. It does not apply to anybody who is shielding.
For example:
A grandparent who lives alone would be able to form a support bubble with one of their children, which means they could go to see them, interact with their grandchildren etc as normal
A single parent could form a support bubble with a parent or friend so they can interact as normal
Two single people who each live alone could form a support bubble
A couple that do not live together could form a bubble - but only if they both live alone or one of them lives alone and whoever lives with the other one doesn't form a bubble with anyone else
So an adult child living with parents could form a bubble with a partner as long as they live alone and the parents don't for a bubble with anyone else.
It's a little complicated when you start talking about every possible scenario (and no doubt there will be many come out with reasons they are special cases and need more than one other household) but actually it makes sense as a first step.