I am struggling to figure out how the Email Action works in Lightning. In the attachment:

• Image 3 is the form we currently have.
• Image 1 has a footer toolbar that includes various tools include the ability to select a Merge Field.
• Image 2 includes a toolbar with additional formatting options.

Does anyone know:

• What determines which Email features are included when using deploying an Email Action in LEX?
• I am most interest in the ability to include the 'Merge Field' icon in the lower toolbar.
• Is there any detailed documentation on this?