Each time a Job is executed this is called an instance of the job; the instance is represented in the API by the ExecutionHistoryItem. That is, each ExecutionHistoryItem contains information about a single, specific execution of the job. 

The job's HistoryRetentionOptions and HistoryRetentionLimit determine how long an ExecutionHistoryItem is retained before being automatically purged. 

ExecutionHistoryItems cannot be created or modified through the API (all ExecutionHistoryItems are read-only). They are created only by the adTempus engine. Use the job's GetExecutionHistory method to retrieve ExecutionHistoryItems. 

The data is retrieved from the server when you call GetExecutionHistory for the job. If the instance is currently executing and you subsequently need to check the status or other information, call the Refresh method first to update the object with the latest information from the server.