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This single event that summarizes many events is refereed refered to as a 'synthetic event'.
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In this example we are stating that all attributes of the first matching rule will be cloned into the synthetic event unless the enrich directive states otherwise. Please refer to the related links for more information about the 'copy_*' options.
Autoacknowledge
This value relates to the matching individual node event. If this value is 'true' or '1', then it will automatically acknowledge the matching individual event causing processing to stop for it. If autoacknowledge is not set in this manner then the individual matching events will not be suppressed.