When a sublist on a record is based on a custom record and the sublist is set to be editable, you may notice that in the view mode of the parent record, a “Remove” link appears next to each line of the sublist.
This happens because the relationship between the parent record and the sublist line (custom record) is not enforced as mandatory. As a result, the system allows you to remove the child line from the parent.
To prevent the “Remove” link from being displayed, you need to make the parent record linking field (the field in the child/custom record that references the parent record) mandatory. By making this field required, NetSuite ensures that the child record must always be linked to its parent, and the option to “Remove” will no longer appear in the sublist lines.