Standard Layouts vs. Custom Layouts
Standard layouts are the pristine model layouts that you can't edit. Custom layouts are copies of standard layouts that you make, which you can edit.
Standard layouts exist to make your upgrades seamless. When you upgrade to a new release, Oracle upgrades only the standard layouts for each object. Your custom layouts aren't touched. This makes it easy for your users to continue working immediately after an upgrade. In the meantime, you can take your time to review the changes that happened to standard layouts as part of an upgrade, and manually incorporate those changes as and when needed.
After an upgrade, you can easily review the newly upgraded standard layouts by deactivating all existing custom layouts for a page type, such as the creation page. Then, sign in as a user to view the standard layout at runtime. Observe the changes for the creation page and, if desired, go back to Application Composer to incorporate those changes into your creation page custom layouts and reactivate them.
If an object has one or more custom layouts for some page types, but not for others, then Oracle considers the whole set of pages for that object to be modified. As part of the upgrade, Oracle:
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Doesn't touch the custom layouts that already exist, as usual.
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Creates custom layouts for those pages that don't have any custom layouts. These new custom layouts preserve what users experienced before the upgrade.
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Upgrades all standard layouts, as usual.
Since standard layouts are model layouts that you can't edit, this means that you can't make changes to the page using Page Composer if the standard layout displays at runtime. However, customers can still personalize the page.