DDS = Data Description Specifications
This "programming language" is used to define the layout of files, both physical and logical, interactive and prompt screens, reports, and various other kinds of objects in the 400 family.
The objects created by DDS do not DO anything like a program, but rather they are layouts for the data to be processed by a program.
Think similar to SDA which is one way that menus, screens, and so forth were created prior to DDS. Prior to RPG/400 we did not use externally defined print files, but internally defined then within the programs that were generating the reports.