If I’m understanding your question correctly; you are asking if you need a letter from your bank to your bank that you are only doing the temporary (or construction) financing.
I don’t think you are required to give yourself a letter stating that you are doing only the temporary financing. That being said I think that it would be prudent to document your loan file with the loan situation. That can be done via a letter, memo, or even an officer comment sheet, etc. That along with the commitment (aka take out) letter should make the situation and expectations clear to anyone who then reviews the file.