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At the top level, the Structured Data Dashboard, which is under Optimization, aggregates this data (by root item type and vocabulary schema). Root item type means an item that is not an attribute of another on the same page.
It also provides per-page details for each item type.Google parses and stores a fixed number of pages for each site and item type. They are stored in decreasing order by the time in which they were crawled. We also keep all their structured data markup.
we have a details page showing all attributes of every item type on the given page
why in the ### would you need or want to see a preview of your own page?