Versioned layouts make it possible to specify different layouts for different versions and languages of the same item.
You use versioned layouts when you want to:
Have different layouts for different languages.
Publish a specific version with its own layout for a specific period.
Sitecore uses versioned layouts internally for various content-testing features, for example, to provide cross-language testing.
In the Standard fields of a versioned item, in the Layout section, two fields are for layouts:
__Renderings – a shared field where you specify the common layout for all languages and versions of the item.
__Final Renderings – a versioned field where you specify individual layouts for languages and versions of the item.
The layouts are stored as XML in the fields, and the contents of these fields are merged ("patched") together to create the final presentation that visitors see.
The following flow diagram shows in detail how Sitecore resolves which final layout to use for an item: