Yes, exactly. If one post has more than one category associated, then the search will show the post as result, unless all of the categories are ecluded, that are associated with the post.
For example:
The post “Test” is in “category 1” and “category 2”. If you exclude “category 1”, then the the “Test” post will still show up, because “category 2” is not excluded. However if both “category 1” and “category 2” are excluded, the post will never show up as result.