Reach is the number of unique followers who received a timeline delivery of at least one tweet/post regarding a particular hashtag.

So, for example, if two tweets are sent by the same person including the same hashtag, then for that hashtag Timeline Deliveries = 2 times Reach.

As with Timeline Deliveries, there is no way to know how many of the messages were viewed. Because the same users may receive multiple timeline deliveries, measuring Reach as well as Timeline Deliveries allows you to better understand the true spread of your message.