A diesel engine welder generator set is a single machine that combines:
A diesel engine driving a generator (alternator) so it can produce electrical power (AC output)
A welding output (usually DC) so it can act as a welding machine (stick/TIG/plasma)
Thus it can generate auxiliary power (e.g., for tools, lights) and perform welding operations, either separately or simultaneously (depending on model) as needed. For example, one description: “Diesel Engine Driven Welder/Generator ... Delivers up to 400 A of FC weld output. Three‐phase and single‐phase auxiliary power available.”