The trackAllContentImpressions() Method

According to the Javascript API reference: “trackAllContentImpressions() - Scans the entire DOM for all content blocks and tracks all impressions once the DOM ready event has been triggered.”

Does the term “content blocks” refer only to that content that has been tagged with the content block attribute or content block class, or does it refer to all visible content and their associated URLs?

Also, is it true that there can be only one content-name per content-block?