When can I expect multiple "choices" in an OpenAI response?

For a simple OpenAI api call in python

response = openai.ChatCompletion.create(    model="gpt-4",    messages=[        {"role": "system", "content": instructions},        {"role": "user", "content": my_input}    ])

I get meaningful data in response['choices'][0]['message']['content']

Under what circumstances will there be multiple entries in response['choices']? How should I interpret this?