After identifying the target audience and learning about the requirements of your target journal, the next step is to choose the most appropriate article type. Articles that are published in the scientific literature can generally be classified into primary research articles and review articles. Primary research articles usually include original research results that have not been previously published. There are many types of articles that fall under this category, and to complicate things, different journals have different names for what is essentially the same type of article.
Full papers (or articles, or original research articles) contain a complete body of research on a particular subject. Therefore, they can be long and contain a significant amount of research.
Letters (or communications) are reports of significant advances that should be published urgently. They are usually shorter than a full paper.
It is important to decide the type of paper that is most suitable for your work. Once you have published an article on a particular topic, it will be much more difficult to publish a similar article without making significant new discoveries. Full papers that mainly expand findings that were previously published as Letters without greatly expanded the scope and without providing new insights or conceptual breakthroughs are usually rejected. Full papers that are mainly routine extensions of previously published related work will also be declined with the recommendation for submission to more specialized journals.