I don't think anyone really knows what the DPA means, and I used to work in areas of the military where the DPA was taken quite seriously. It covers every area about every conceivable piece of information and the couple of odd times I tried to read the regulations, it just made my eyes bleed; it's not interesting stuff.
It could be more to do with the storage of information maybe; is there security codes for the building and stuff? Maybe these offices take industrial espionage seriously. It won't be anything to do with your PLI.
I think you ought to just ask who ever asked you about the DPA.