Casual Leave - is designed for emergency reasons or unforeseen occasions, such as attending social functions like weddings or birthdays or fulfilling personal tasks like banking, etc.
Sick Leave - is designed for individual sickness. If an employee avails of this leave, some companies may require medical certificates or fitness certificates.
Earned Leave or Privilege Leave/Paid Leave - is designed to provide rest or recuperation for short periods, such as holidays. CL/SL cannot be accumulated, while PL/EL can be accumulated according to company policy.
In the absence of CL, an employee may use SL to meet their social needs. Similarly, in the absence of SL, an employee can use CL or PL during their sickness. Finally, PL/EL cannot be substituted with other types of leave, and they cannot be combined.
Please note the corrected version above. Let me know if you need further clarification.