CONSTANT:
A constant force will keep the same level in time. When a game decides to apply a force of x% of what the wheel can do, the "Constant" will keep that force at the same level.
Example: A driving simulator game will usually use the constant force to simulate the G-Force. It will create a constant force at 0% but, depending on the speed and the wheel angle, the force will be increased accordingly.
PERIODIC:
A periodic force will vary in time according to the type of periodic effect, amplitude and frequency. Because a periodic force includes an offset that has the same behavior as a constant force, some games will create a periodic force and merge an effect they would render for a constant force (like the G-Force) and a periodic effect (bumping road).
Example: A periodic effect gives the shaking effect on the wheel. At high frequency, you will feel it rumble... but in some cases very low frequency are used to create crash effects where the wheel first turns fully to the left, then to the right and then to the left again, before stopping. Of course, that depends on what the game developers decide.
SPRING:
A spring force is a force that increases according to how far you are from a specific position on the wheel. The basic spring force we could think of is a default spring center where the wheel goes back to its center position when you release it. But the force can be set to negative, which will make the wheel go further away from the center position.
Example: Basic spring force where the wheel goes back to its center position.
DAMPER:
A damper force controls how the wheel will react when it's moving. It is usually used as a (dynamic) friction or if you use it while the wheel gets back to its center position (spring effect), then it will behave like controlling the damping on a spring-mass system. A game will usually use the damper force in order to make it harder for the player to turn the wheel while in other circumstance make it feel like if it's very smooth and easy.
Example: Controlling the force you need to apply on the wheel to rotate it.
The SPRING and DAMPER effects are what we call Dynamic Effects, because they rely on information which depends on the wheel (position for spring or speed for damper). The Dynamic Effects are those effects which benefit the most from hardware force-feedback implementation (compare to software) - like in the T500RS - because you want a fast response based on how the player will turn his wheel.
The CONSTANT and PERIODIC effects are what we call Static Effects. Once the game sets them, they act according to the parameters set or modified through time by the game regardless of how the user turns the wheel. The game might adjust them accordingly, but they are not linked directly to the wheel position, speed or acceleration.
