Thank you very much for your reply.
I have a follow-up questions to be more clear.
1) If downloaded software is regarded as physical product, what is the tax rate for the sales of downloaded software under tax treaty between ths US and Korea?
2) What is the reason that Apple says Korean developers do not need to hand in EINs in W-8BEN form and does not withhold tax when Korean developers do not fill in EINs?
Thanks for your help!