Types of Zoho Projects Users

Types of Zoho Projects Users

When managing your Zoho Projects Subsription to add "users", there are 5 options:
  1. Additional Users
  2. Client Users Add-On
  3. Support Add-on
  4. Readonly Users Add-On
  5. Resources Add-On

No where that I have found clearly describes the differences between these 5 types of users.  Can someone please clarify?