In British schools, CDT is a practical subject: Craft, Design and Technology. Students learn to design things and then make them with tools — wood, metal, plastic, and later electronics too. “Practical” here means you do not only sit and read: you plan, measure, cut, and build. The name CDT was common in the late twentieth century; many schools now call the same area Design and Technology, or D&T, but older exam papers and parents may still say CDT. It is a UK school name, not a world-wide one. Taiwan’s closest school subjects are 生活科技 and the older 工藝課, but they are not called CDT. Though actually, in American English the same three letters almost always mean something else: Central Daylight Time, a summer clock setting in the middle of the United States. If you see CDT next to a clock or a TV schedule, that is the time zone, not the school subject.