To control the rendering of unique and default content based on the presence of subscriber data in a related data extension, the best practice is to use theRowCountfunction and anIFstatement:
%%[ VAR @rowCount SET @rowCount = RowCount(LookupRows("RelatedDataExtension",
"SubscriberKey", _subscriberKey)) IF @rowCount > 0 THEN /* Render unique content */ ELSE /* Render default content */ ENDIF ]%% This approach checks if there are any rows in the related data extension for the subscriber and conditionally renders the appropriate content.
References:
* AMPscript Guide
* Salesforce Marketing Cloud Documentation