Correlation tokens are unique identifiers in SharePoint that bridges to map the objects between the workflow and windows workflow environment. Windows workflow acts as a intermediate layer to communicate with other software. When incoming request arrives, runtime receives the request and determines the request and identifies the workflow instance. Then it process the request for the instance.
Windows workflow acts as a communication channel for all the workflow instances. We can have two separate correlation tokens for Workflows and task references. We cannot assign the correlation token to workflow task that used for workflow.
We have to declare the correlation token for the workflow in OnWorkflowActivated activity. We have to bind the this correlation token to each activity in the workflow.
OnWorkflowActivated activity raises when workflow initiated for SharePoint. This is the first event raised in workflow. Each task should have its own correlation token that provides the task related information.