You mean the letters and numbers like A8-3. In this example, the letter indicates the total impulse or total power produced by the engine. Each succeeding letter has twice the power of the previous letter, for example, ‘B’ engines have twice the power of ‘A’ engines. The first number next to the letter (in an A8-3 engine, this would be the number 8), shows the engine’s average thrust in newtons, or the average push exerted by the engine. The second number after the dash give you the delay in seconds between the end of thrusting and the ejection charge. This lets you choose the delay time you want for any flight. Engine types ending in ’0′ have no delay or ejection charge and are for use in booster stages only.