DDS is coded in source file members like any other piece of programming code. IBM suggests you use source physical file QDDSSRC in whatever library you are programming. You don't have to, however; if you feel better having one source physical file (call it SOURCE), you can do that too.
The source type depends on the type of file for which you are coding DDS: PF for physical files, LF for logical files (including join logicals), DSPF for display files, ICFF for interactive communications file, and PRTF for printer files.
SEU supports DDS. For example, SEU has predefined prompts for entering DDS. These prompts are: PF (physical files), LF (logical files), and DP (display or printer files).