Display toggle options appear as you click Options in the USD Layer Editor.
Each USD stage is created with a session layer which provides a "scratch space" to configure, override and experiment with layer edits. To learn more, see session layer in the Pixar USD Glossary.
When enabled, you will get a prompt when saving existing .usd files (note that this is not for export and only valid for core USD files). If disabled, you will not get a prompt.