A configuration schema can contain not only configuration objects, but also complex defined types. For example, in the core.data_types.schema.yml file there is:

# Mail text with subject and body parts. 
mail: 
  type: mapping 
  label: 'Mail' 
  mapping: 
    subject: 
      type: label 
      label: 'Subject' 
    body: 
      type: text 
      label: 'Body' 

This allows any other configuration schema to define its type as Mail and have it interpreted as that mapping without having to repeat it.