@Liza *****

I don't know it yet, I'll use the original recommendation letter from university to cancel as soon as I arrive at the immigration office of Chiang Mai.

I didn't need to cancel it because it was "used". I need to cancel the extension of stay with ED visa. That's all. Visa and Extension aren't same.

@Sascha *********

Yes, I also think as you said. BUT it was not like that. My ED visa was used once I entered Thailand (there is a written "used" on visa sticker) which means I only have an extension of stay with re-entry permit, not a valid visa.

@Sascha *********

the cancellation letter is the recommendation letter from the university to cancel the ED visa and signed by the principal.

This is my opinion and not facts i don't know for sure. I think the visa section and extension of stay permit is in a different operation. For example - if you're on a visa which is not stamped used in passport means you can't legally apply for another visa type. If your visa stamped used then you do an extension of stay. That means you're on an extension of stay. So if you apply for another type of visa the visa section may not find that you're on an active visa. You might get a DTV or some type of visa. The rules might be changed anytime so the safety way is to cancel the visa or extension before you apply for another one.

@Greg *******

yes, you're right. They're asking where I will go next. That does not necessarily need information. And plus i was quite late to check in and they look at my passport and like they don't see my visa for Thailand. What do they mean like that there's a clear sign that there's an ed extension and i was there for like a few minutes. Anyway the immigration doesn't even say anything Tbh.