The File menu allows the addition to the current document of pre-defined composites. The first time a built-in composite is added a root-level 'Composites' container is created. Any subsequent addition go in the same container.
If a built-in composite is added to a document in which the default value of $NeverComposite is true, the components of the built-in composite have $NeverComposite set to false, ensuring the newly added composite works correctly. If Tinderbox needs to create the 'Composites' container, it sets the OnAdd action to $NeverComposite=false to ensure any user-created composites work correctly when added there.
The OnAdd actions for the built-in containers for Prototypes and Templates set $NeverComposite=true. This reduces the likelihood of unwanted accidental composites.
The built-in Composites container is set to export neither itself ($HTMLDontExport) nor its children ($HTMLEXportChilren). This can be verified in the Export Inspector.
The built-in composite types are: