Introduction
The question assumes scenario in which tens of XHR requests have to be managed and occasionally a sent request needs to be canceled.

Analysis
The RequestManager object provides the cancel method with the request object parameter. One would think that, if sender-client saves sent request object, request could be later canceled by calling the cancel method and supplying saved request object for parameter.
However, sent request object is modified internally by RequestManager, e.g. by adding or changing the age, priority and transport properties. The sender-client is not aware of such modifications and cannot supply a request object the cancel method needs to perform desired cancellation.
My conclusion is that the cancel method in current RequestManager version cannot be called from outside of RequestManager itself.

Question
Is above conclusion correct? If it is, would assigning some unique identifier to every sent request and returning the identifier to sender-client make cancellation possible? This would imply replacing references to request objects with references to request identifiers wherever needed.

You're correct, the RequestManager was written to manage the cancellation for you, not to allow you to cancel the requests manually.

Your proposed approach to extend the functionality would work, and in fact, is the way that many Ajax libraries work.

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Nicholas C. Zakas
Author, Professional JavaScript for Web Developers (ISBN 0764579088)
http://www.nczonline.net/