Answer:   Assignment rules are used to automate your organization’s lead generation and support processes.
Lead Assignment Rules – Specify how leads are assigned to users or queues as they are created manually, captured from the web, or imported via the lead import wizards.
Case Assignment Rules – Determine how cases are assigned to users or put into queues as they are created manually, using Web-to-Case, Email-to-Case, On-Demand Email-to-Case, the Self-Service portal, the Customer Portal, Outlook, or Lotus Notes.
Aim: All the cases created by user ‘ABC_User’ should be assigned to ‘XYZ_User’.