What is a e s s letter?

An "e s s letter" typically refers to a letter that is part of an abbreviation or acronym, but the term itself is not widely recognized. It could possibly relate to specific contexts, such as a type of communication, a designation in a particular field, or a letter format used in certain organizations. Without additional context, it's challenging to provide a precise definition. If you have a specific context in mind, please clarify for a more accurate answer.