The oddly named Development Pac was a cartridge for doing assembly language programming.  According to the manual it contained several programs including a Z80 assembler, relocating linking loader, line editor, I/O routines, and the DDT80 operating system debugger.   This cartridge was used by serious programmers who needed a language that was faster than the high level BASIC that also came with the system.  The Development Pac retailed for $99.