What does "update frequency" mean/do?                                     
                                        Author: Anna-Maria Andersson
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In the site map, page properties window at the bottom you can set the "update frequency" what does this mean/do? What is an optimal setting?
Posted on the 

These settings are for optimization for search engines (see top of screen)
This is for search engines to have an idea about the time's the page's change.
So they can revisit and index your site correctly in there database.
BR
A