This is Google’s definition of domain:

“A specified sphere of activity or knowledge.”

Everything within a codebase that represents a section, module, or resource must have a folder created for it. Here are some examples:

• Sections (what’s in the lib folder of an application)
  • lib/common
  • lib/core
  • lib/l10n
  • lib/module
• Modules (what’s in the lib/module folder)
  • lib/module/friend
  • lib/module/game
  • lib/module/home
  • lib/module/invitation
• Resources (folders that actually have code)
  • lib/module/friend/creation
  • lib/module/friend/delete
  • lib/module/invitation/dto
  • lib/module/invitation/repository