If you have Advanced Security activated in your account, you will see further options for you to add Security Groups to a record.
In the Action Type List you will have another option of 'Set sub tab security template'.
Fig 1 Advanced Security Group option
This option will now allow you to set detailed rules as to what Security Groups both your header record and its sub tab record will have.
Inherit from the header - This option is selected by default if no change is made and the pop up is closed. All Sub Tab records, new or existing, will inherit the Header Security Group.
For all sub entities - If you wish to apply a Security Group different to the Header to all the Sub Tabs then choose this option and select the Security group from the drop down list, this will change all Sub Tab records at once.
For individual sub-entities - This option allows you to select a specific Sub Tab option from the drop down list and then specify if this Sub Tab's Security Group should be passed down from the Header, or using 'Apply' and the drop down list you can assign an alternative Security Group for that one specific Sub Tab.
Note: For more information above Security and sub tab behaviour please read this article https://support.intelligentcontract.com/support/solutions/articles/22000265988-record-and-sub-tabs-behavior-using-security-groups