Correlation tokens are something very important in SharePoint
A correlation token is a means of uniquely identifying each instance of a workflow, modification or task. When SharePoint initiates a workflow, it does not spawn a unique set of objects. Instead, if one instance of the workflow is already running when the second initiates, the second will reuse objects from the first. Correlation token properties ensure that the single activity object is operating on the correct workflow instance and accessing the correct details about the workflow.