Why do you care? Because in ITM 6.1, the events are handled differently and may require different operator actions.
So what is a Pure event? Think of this way - an appliance on your network writes an entry to a logfile that something happened, like an HP printer with a toner low message. This is pure, there will be no clearing event - it just happened and as such you must manually close these events in the Tivoli Enterprise Portal and the Tivoli Enterprise Console.
A Sampled event is just that, a metric was sampled from a server - like Free Memory. That metric was compared against a threshold that you defined in a situation. When the sampled data exceeds the threshold, an action and/or alert is generated - when the sampled data falls below the threshold, a clearing event is generated.