Glossary
What Is Reach vs Impressions?
Reach is the number of unique accounts that saw a post; impressions count every time it was shown, including repeat views by the same account. So impressions are always equal to or higher than reach. Some networks now report views instead of impressions, and each defines them slightly differently.
Updated
Example
If 100 people each see a post twice, reach is 100 and impressions are 200.
In Markaestro
Views, impressions, and reach are stored separately, exactly as each platform reports them; which ones a network returns is listed on each channel page.