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The Descriptor is a text field that appears on various client application displays and can be used in reports. It can be of any length up to 65,535 characters. When this value is read through the PI API it is truncated to 26 characters. But Processbook uses the PI SDK. I also found this internally.
"I tried creating a descriptor of 5k characters and was able to get it completely in Excel 2010, SMT, and ProcessBook."