So...What is the difference between "Rank" and "Grade"?

Grade and rank are often used inter-changeably and are also often confused. Grade is a major step in the promotion structure or program while rank is grade adjusted for time. “Captain” or “major” are examples of grade; several individuals can have the same grade. Rank refers to seniority between two persons of the same grade such as date of rank (no two persons in a grade have the same rank - one is always senior to the other). Bottom Line: The major difference between grade and rank is that grade is a major step in the promotion process. Rank is seniority between two people of the same grade.