The rail the form will show, resolved the way the form itself resolves it.
Whether the form shows a side rail. In a rail layout the chrome bundles every field section into one "Details" tab, so a user reading the section list sees names that are a level below anything the form shows them.
Every panel the form drew, including the related grids. Feeds the rail.
Field sections the form drew, in document order, each with the inputs it draws. Replaceable a section or a single field at a time.
What a rendered form turned out to contain.