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In Microsoft Excel under Reviews > Comments there is button which says “Show Ink”. This basically is used to display hand written notes on excel file using stylus or any other device created on Tablet PC. So when a user reviewing an excel document want to add a hand written comments he can use an input device and the system captures and saves it into the document at the right place.
I was able find one sample from here which gives you a view of how an Ink Annotation looks like :