This is intended to be a new sub-page in the User Guide, at User Guide > Using Backdrop > Layouts and templates > Deep dive: advanced layout options > URL paths, which would come after Visibility Conditions and Contexts sub-pages.
Layout overrides are quite powerful because a single layout can be used on multiple paths if its primary path is dynamic (i.e., contains a placeholder "%"). However, once we have defined the primary path for a layout, it won't be used on any paths whose menu router path is different. Fortunately, layouts provide a setting, "URL paths," that can allow a layout to be used on many different paths.
URL paths are a powerful feature of layouts, but they come with some new concepts and require some subtle distinctions that we have not previously delved into. So we begin this section with some useful background information.