"Subtabs
of" in Salesforce Lightning and "As a subtab of " in Salesforce Classic
is used to decide how records display when they are selected from
outside of primary tabs or subtabs.
For example, when users select cases from list views, search results or screen pops, you can select cases to display as primary tabs or as subtabs on parent records, such as accounts.
For example, when users select cases from list views, search results or screen pops, you can select cases to display as primary tabs or as subtabs on parent records, such as accounts.