The definition depends on what the tool does. For a drafting tool, coverage is usually emails with a draft divided by emails that needed a reply. Emails deliberately skipped, such as newsletters and notifications, belong in neither number.
Why it matters
Coverage is half of the picture. High coverage with poor drafts means a lot of editing; low coverage with excellent drafts means most of the inbox is still written by hand. Read it next to a quality number such as the sent-unchanged rate.
It also shows gaps. If a whole kind of email never gets a draft, it may be filtered out by mistake, or the tool may lack the sources to answer it.
An example
Two hundred customer emails needed a reply this month and the tool drafted one for a hundred and eighty. Coverage is nine in ten. The twenty without a draft were mostly questions in reply to an old thread, which is worth looking into.