That's by value. I reckon since Chinese imported goods tend to be cheaper they probably make up a greater number than the value percentage would indicate.
So I suspect 40% figure is how many items in a typical household are from China.
Edit: in case that's not clear, here's an example:
I have one item from the USA that costs $80. I have four items from China that cost $5 each.
My imports from China are 20% of my spending. My imports from China are 80% of my goods.
It is clear what you are saying. It still doesn't make any sense.
If I buy a bottle of wine, a chicken, a cake, and a pint of blueberries, is it correct to say that my diet is 99% fruits? There are more than a 100 of blueberries in the pint after all.
So I suspect 40% figure is how many items in a typical household are from China.
Edit: in case that's not clear, here's an example:
I have one item from the USA that costs $80. I have four items from China that cost $5 each.
My imports from China are 20% of my spending. My imports from China are 80% of my goods.