Occurs when a DML operation that is intended to overwrite only part of a large column value is forced to rewrite the entire ...

Occurs when a DML operation that is intended to overwrite only part of a large column value is forced to rewrite the entire value.  This happens if the table's heap or clustered index is being built online.  Use this event to identify DML workloads that are running more slowly as a result of building an index online.