These designations indicate the level of responsibilty and seniority for each. For example, If you have 4 sergeants in a unit, the Sgt. with the most seniority is usually the 1st Sgt. The 1st Sgt. has administrative responsibility for the company. Th sergeant next in line by seniority and responsibility was the 2nd Sgt., and so forth. The same goes for Corporals. Thankfully this complicated system no longer exists. A 1st Sgt. is still the company's administrator but the other levels of Sgt. are no longer indicated in rank designations.