Check the third party invoice in column 13 of form e

We have a prospective client from Myanmar who is now consulting us about the form e certificate of origin. They have imported goods to Myanmar in our country before, and they have reflected the form given by their previous suppliers The goods description in column 7 of e certificate of origin shows the title of their supplier mainland company, so the third party of item 13 can not be checked. I see in many places on the Internet that only offshore companies can check the third party item in column 13, and also need to provide an offshore invoice. However, I can see that you sent a message saying: if it is a mainland company doing form on your side E certificate of origin, you can check the third party option in column 13. Is it true? If a third party is checked, will our customs clearance information be the same as what we normally do in the past? Will this affect customs clearance or tax refund?