When creating an extended table in ServiceNow, it inherits all fields from the base table, meaning you do not need to recreate those fields. Instead, you only need to define fields that are:
Not present in the base table - If a field does not exist in the base table but is required for the extended table, it must be created.
Specific to the extended table - Custom fields that are unique to the child table need to be added.
Why Other Answers Are Incorrect:
A . The mandatory fields for the base table - Since the extended table inherits the base table's fields, mandatory fields from the base table are already included automatically.
B . The reference fields for the base table - Reference fields are not always required unless they are specific to the extended table's functionality.
Example Scenario:
If you extend the Task table to create a Custom Task table, the new table automatically gets all Task fields (like Number, Short Description, etc.).
If you want to add a new field, such as Custom Priority, you need to create it manually.
Reference from Certified System Administrator (CSA) Official Documentation:
ServiceNow Extending Tables
ServiceNow CSA Training Module: "Data Schema and Extending Tables"