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Update is similar to publish, except this is for editing existing pages. Existing pages may contain edits in the current draft, which will also be included when updating. Update provides additional publish options such as whether or not a notification gets sent to watchers, date/time publishing, and including page version comments.
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Drafts
Drafts allow you to work on content while not affecting what other viewers of the space can currently can see. A page can only have one draft, and all edits or updates made by all users go towards the same draft until it is published.
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