"NULL_OVERRIDE " entry in sys_choice tableIssue If you will go to sys_choice table , you will notice that there are plenty of "NULL OVERRIDE" value present . This "NULL_OVERRIDE" is used to change the default display label of the None option for a choice field.When you choose none as the display value ,You must enter NULL_OVERRIDE as the value, or the new label appears in addition to the -- None -- option.This is the use of this entry.So it is advisable to not to make it inactive as it has some feature to offer and may be helpful.This is the documents that will give you a better idea about this:https://docs.servicenow.com/