Basically, you run the article through Copyscape. Copyscape will return a number of results that matches your article. So you can just click the first result.
And you will probably see something like this:
The page below has 80 words matching 6% of the page, as highlighted below:
This means the article is 94% unique when compared to the first result.
When you click on the next result, you will probably see something like this:
The page below has 67 words matching 5% of the page, as highlighted below:
So compared to the second result, it is 95% unique.
To answer your question, it is uniqueness regarding the overall article. And yes, all individual sentences will match Google results if looked up on sentence basis. (unless not a lot of people are using a particular category in Article Builder)