“Letter to the Editor” is considered a “post publication peer review”. They are generally listed in the scientific databases as a publication. I think so.

The actual term is Research Paper which consists of 1. Original paper 2. Review Article. 3. Short Communication. 4. Research Note. 5. Letter to the Editor. 6. Corrospondance 7. Data base or methods etc. It depends on a policy of a specific journal that the nature of manuscripts they publish. So publication in any form should be considered as research paper.

Thank's for clarifying the role of the "Letter to the Editor" as a research paper, dear Drs. Abu Reza Towfiqul Islam and Arijit Das.

But what about letters to colleagues commenting particular publications of theirs as such which were presented on https://medium.com/@franzschelling?

Or such a booklet such as commented on below?