On our Stats page within your dashboard, and in a few other places we mention the Total Reach and Unique Reach of your social posts, and we understand that it can be confusing to see two different Reach numbers.
First, Reach is always calculated from the number of followers a person has. This does mean that for some social networks where they don't share details about the number of followers a person has with us, that we'll report no increase in reach when they have a post coming in (Instagram, SMS, and Web Form will all report 0 followers).
Second, for the content sources that do share follower details, the difference between Total Reach and Unique Reach is that Total Reach counts the followers each and every time that a user sends a post, however Unique Reach only counts them once per day.
The example we like to use is that if I have 1,000 followers and send three Tweets on Twitter, Total Reach would increase by 3,000, but Unique reach would only increase by 1,000. Some people refer also to Total Reach as Impressions or Absolute reach.