This is what IEEE writes in this regards: "Journals, Transactions, and Letters are the primary means for publishing technical papers concerning original work in IEEE fields of interest. Journals, Transactions, and Letters may also contain reviews and tutorial papers concerning such work. The primary purpose of Journals, Transactions, and Letters is to disclose and provide a permanent archival record of original technical work that advances the state of the art or provides novel insights. Papers in Journals, Transactions, and Letters should be of lasting value to the professional as judged by the authors' peers through a formal review process. Letters are for the publication of brief papers, usually three to four pages in length. Magazines are characterized by regular and continuing issues with significant technical content in addition to general news and regular columns." Is there any other information that could be useful? How to know whether a paper should be labeled as Transaction or magazine?
See these discussions
https://academia.stackexchange.com/questions/13960/difference-between-transactions-and-journal
https://www.quora.com/What-is-the-difference-between-transactions-and-journals-ACM-IEEE
There I have found the link to the following document
https://www.ieee.org/documents/auinfo07.pdf
which states that
IEEE Transactions generally contain major manuscripts approximately 8 to 10 printed pages or 24 to 30 double-spaced pages.
IEEE Journals follow the same length criteria as Transactions, but often are focused on selected topics and more specialized areas of interest.
IEEE Letters are generally short papers of approximately three to four printed pages or nine double-spaced pages.
Also, perhaps this tool could help you in choosing the right venue for your papers:
publication-recommender.ieee.org/
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