A page can hold up to 200 products, with their variations listed under them (up to 500 variation rows a page). Saves are sent in batches of 20 rows, so a full page of changes is ten quick requests rather than one long one, and each batch goes into History as it is saved. To change one column on every record a filter matches, across all its pages, use Apply to: All N records in this list in Calculate, Set all selected or Find and replace. On very large catalogues, filter first; the grid renders lazily so even wide rows stay quick.
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