Publication Approval Letter I will publish a conference paper and as I'm a PhD student the committee asked "If the corresponding author is a PhD/MSc student, he/she also should send a Publication Approval Letter from his/her supervisor".

What does it mean? What is a Publication Approval Letter?
Judging from your comment, this publication approval letter is supposed to show that your supervisor approves of the publication (i.e. that your supervisor is aware of the publication and has no problems with you publishing it). You'll have to ask your supervisor for it.