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The Placeholder module is always needed in the Page Base. It denotes the place in which content of pages created with the Editor, product details and category pages will be displayed. So everything you will set up in Category Base and Product Base will “go into“ the Placeholder. You don’t need to drag and drop the same set of modules on these bases again.
On most websites everything above the Placeholder will be the header while everything below, the footer.
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Triggering page re-rendering
Once you are done with your base updates, click the Save&Re-render button top right. This will start updating the page .html files for all pages using this specific base, so:
Page Base - will trigger re-rendering of all subpages
Product Base - will trigger re-rendering of all product details pages
Category Base - will trigger re-rendering of all category pages
This might take more than a couple minutes, especially if you have thousands of products in your BigCommerce inventory.
Alternatively, you can click the Save button top left and not trigger the re-rendering process.