W-2 Form for International parents?

On my IDOC page, my parents were requested to upload their W-2 form and 2015 federal tax return.

But since my parents earn money outside of the US, they don’t have the W-2 form. What should they do? Should they just click “Click here if you did not, and do not expect to, receive a W2 for this year” and submit a substitute document for W-2 form?

Yes. Do they have tax documents or records of any kind from another country? Are they self employed so would have other statements of earning similar to a 1099?