Hi RJE,
The difference between a source and a sink output is whether it provides a ground or voltage source. So, depending on your circuit, if you need a ground then select the 6515 for its sink output, and if you need a voltage source use the 6514.
Here is a good knowledgebase on the topic.
Understanding sourcing and sinkingHope this helps!
Jeremy L.
National Instruments
Jeremy L.
National Instruments