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LookupRows('Exclude', 'SubscriberKey', _SubscriberKey) retrieves rows whose SubscriberKey matches the sending subscriber. RowCount(...) > 0 evaluates true when at least one exclusion record exists. A primary key permits at most one matching record, so testing for more than one row would not identify an excluded subscriber. Salesforce documents that LookupRows() returns a rowset matching the supplied criteria and that RowCount() returns the number of rows in that rowset.