no expert, but from my messing around I gather the chat models are tuned for conversation, for example, if you just say 'Hi', it will spit out some 'witty' reply and invite you to respond, it's creative with it's responses. On the other hand, if you say 'Hi' to an instruct model, it might say something like, I need more information to complete the task. Instruct models are looking for something like 'Write me a twitter bot to make millions'... in this case, if you ask the same thing again, you are somewhat more like to get the same, or similar result, this does not appear so true with a chat model, perhaps a real expert could chime in :)
