In computing and office talk, DP stands for data processing: taking facts — numbers, names, records — and running them through machines so they become useful lists, totals, files, or reports. You meet it in older job titles (DP manager, DP department) and in school or business writing from the years when computers first entered offices. Though actually, people today more often say IT, computing, or just 'data.' DP in this sense is a bit old-fashioned as everyday speech, but it still turns up in textbooks, company charts, and history. This is not the only thing the letters DP can mean — elsewhere they might stand for something completely different — but here they mean the computing one.