"To explain the difference an analogy is called for - A room full of people for a business event where perhaps you are talking on a stage or to them directly.
Audience - The people in the room are the audience, they are there and might hear what you say. These people might be in the same room as you but then so are the people you walk past in the street, shopping mall or car park.
Reach - These are the people that will actively listen to you when you are stood at the front of the room sharing your message, whereas the audience are continuing their conversations paying no attention to you at all..."
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