If you ever looked at Excel Services logs on a SharePoint 2007 box (also called ULS logs), you probably saw at the end of some of the log lines a GUID that looked a little like this:
This GUID is used by Excel Services to give information about a specific request. When a request comes into the system, it is assigned a CID (Correlation ID) and that in turn can be used by people looking at the log to divine some information about the request.
Note: It is important to understand that the meaning of this GUID may change between versions. You must not assume that it will stay the same across versions.
The GUID is actually made out of 4 parts:
Request ID: This is a running counter. Each request that comes into the system is assigned with consecutive number. If this number is 4C4B400 (the highest I saw), that means the server has processed about 80 million requests.
User Name Hash: The result of running the GetHashCode() on the string that holds the name of the user running the request.
Signature: This is always the same for all Excel Services requests. It is just a unique, randomly chosen number that can be used to look for correlation IDs belonging to Excel Services.