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.
One of the things to watch out for is when to use a different correlation token - a great starter about this is the blogpost of Eilene Hao about the importance of correlationtokens.
For more information about correlation tokens take a look at :
- Correlation tokens in workflows
- Using correlation in workflows
- Correlation tokens cannot be set at runtime
PS Shameless plug - if you want to learn more about workflow development in SharePoint, definitely buy Real World SharePoint 2007: Indispensable experiences from 16 MOSS and WSS MVPs . Robert Bogue wrote a chapter about WSS workflow development and I did one about SharePoint Server workflow development...