To add a sub tab in a record Navigate to Customization>Forms>subtabs>New.
Click the subtab for the type of record where you want to create a new subtab: Transaction, Entity, Item, or CRM.
This method of creating custom subtabs enables you to quickly create multiple subtabs for a specific record type. You cannot, however, specify your own script ID from this location. NetSuite automatically generates the ID for you. You can later edit this system-generated ID using the Change ID button available on the Custom Subtab page for the subtab.
Create a Custom Record by: Customization>Lists, Records and Fields>Record Types>New
Add fields as required.
In the field select the Record is parent option and under the display subtab select the custom subtab as parent subtab and save.
Result (customer Record):
