The Digesto scheduler sets the frequency at which your RSS feed will be checked to see if there are new posts.
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If there are no new posts, nothing will happen, and Digesto will proceed to check again at the next scheduled time. If there is at least one new post at the scheduled run, Digesto will fire the associated Marketo Campaign and will include all new posts since the last Digest was sent (up to the max number of posts you've set in the Digest).