The Product Dispatch Identifier (PDI) is a unique code that identifies a particular product in an order you have submitted. You can think of it pretty much like you would an ordinary order number. The big difference is that since an order may contain several items, there is a unique PDI for each item in the order. Whereas each order obviously only contains one order number, even if the order is for multiple items.
The PDI is central to the OtsZone system. Everything you can do with relation to a product you have purchased -- upgrade it, increase the license quantity, re-issue licenses, etc -- is all based on the PDI for that product.
An example PDI is as follows: PAAA-C7BT
It contains a mixture of numbers and letters, but is always in the above format of two groups of four characters, separated by a hyphen.
No. You should keep all of your license information private, but having the PDI alone will not allow anyone to do anything to your account.