Model rocket engines are marked with a three character code that specifies the approximate operating characteristics of the motor. The code consists of a letter and two numbers such as D12-5.
The letter is the total impulse, the first number is the average thrust in Newtons, and the second number is the time delay in seconds to the initiation of the recovery system. Hence, the motor in the figure is a class D total impulse engine with an average thrust of 12 Newtons and a time delay of 5 seconds.
The letter indicates the total impulse class of the engine, which is effectively the amount of fuel in the engine. The total impulse is the total momentum change that an engine can impart to a rocket. Total impulse is measured in Newton-seconds (pound-seconds). The standard impulse class for each letter is shown in the following table.
|0.000 - 0.625
|0.626 - 1.25
|1.260 - 2.50
|2.510 - 5.00
|5.010 - 10.0
|10.01 - 20.0
|20.01 - 40.0
|40.01 - 80.0
|80.01 - 160.0
|160.01 - 320.0
Most commercial model rocket engines are built to operate at the top impulse level of their class, but this is not a requirement. An engine may actually operate anywhere in its impulse class range.
|Note: each engine class is double the impulse of the class below it, so as you increase the class of an engine, you effectively double the amount of fuel each contains and double the amount of momentum it can impart to a rocket.
The number following the letter indicates the average thrust of the engine in Newtons (pounds). Because the amount of fuel in an engine is fixed by the class letter, an engine with higher average thrust burns up its fuel more quickly than one with lower average thrust. As a rule of thumb, the duration of a burn is roughly equal to the total impulse divided by the average thrust. Here is a typical thrust profile for an engine with an average thrust of about 6 Newtons.
A typical engine starts with an initial high thrust for a fraction of a second, which is useful for getting things moving. It then settles down and burns the remainder of the propellant at a relatively constant rate. 
The last number on an engine is the time delay, in seconds, to activation of the recovery system. The propellant in a model rocket typically burns up in about 1 second. At that point, the rocket is still moving upward at a high rate of speed. If you were to activate the parachute at this point it would likely be shreaded. What you want to do is to let the rocket coast up to its highest point and then activate the parachute. The time delay charge is the mechanism for delaying activation of the recovery system until the rocket reaches its highest point. The time delay charge also emits smoke to make the rocket easier to track. When the smoke charge burns out, it ignites an ejection charge that activates the recovery system.
|Note: Some larger models use altimeters to sense when a rocket is at its highest point and electrically fire the ejection charge. The Engine's ejection charge also fires a little bit later as a backup to the altimeter.
Time delays are typically 3 to 8 seconds, with short time delays needed for larger heavier rockets and longer delays needed for lighter ones. Do not use too long of a time delay as it may allow your rocket to impact the ground before activating the recovery system. Such impacts endanger the spectators and are really hard on your rockets.
Rocket motors marked with a time delay of 0 seconds are booster engines. A booster engine is used in the lower stages of a multi-stage rocket and has no time delay and no ejection charge. When the fuel finishes burning there is a flash of flame out the back of the engine that is used to light the next engine in a multi-staged rocket. Only the top stage in a multi-staged rocket needs an engine with a time delay and an ejection charge.
Rocket motors marked with a P instead of a number for the delay charge are "plugged" engines. A plugged engine is similar to a booster but the forward end is plugged so no fire comes out the front when the fuel finishes burning. These are used in some gliders and in situations where you do not want a blast out the front.