During the Employer creation process (See Employers > Setting Up An Employer) you will be given the option to select to create the Employer under a Parent Employer.
This is used if the Employer is either a sub-branch of the Parent Employer or if the Employer is required to inherit the insurances from the Parent Employer. You can check the Parent Employer is correct before submitting by clicking on Details.
The option to select "Insurance from Parent Employer" will become available once a Parent Employer is selected.