Each time a Job is executed this is called an instance of the job; the instance is represented in the API by the ExecutionHistoryItem. Each step executed for that instance is represented by an ExecutionHistoryStep. 

If a given step does not execute for an instance, no ExecutionHistoryStep will exist for it. If a step is executed more than once, there will be one ExecutionHistoryStep for each execution of the step. 

The data is retrieved from the server when you retrieve the ExecutionHistoryItem using 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.