The Orbit tab has rotational controls for sprites around their origin. Used in conjunction with Offset. With an Offset of 0, Orbit will not function correctly. Orbit uses Offset to know how large the circle of Orbit is. If Offset is set to 10 in the Forward axis, Orbit will use those 10 units as the radius (not diameter) to orbit the center of the effect by.
A good example effect to look at Orbit is: effects/weapons/rocketlauncher/impact.fx. The sparks segment uses Offset and Orbit to generate swirling and random sparks within the explosion. The larger the Orbit and Offset values, the more chaotic the Orbit will be.
Another example is effects/monsters/gladiator/railgun_trail.fx. The spiral_parts segment uses Offset and Orbit to spiral sphere segments inward. The initial offset values transition from 15 to 1, so that the sprites move inward, while the Orbit rotates the sprites.
Start – Roll, Yaw and Pitch
This adjusts the start angle of the orbit. Example: The min for Start Roll is 40, while the max is 80. The sprites in this segment will start somewhere between 40 and 80 degrees in the direction of the Offset.
End – Roll, Yaw and Pitch
This adjusts the ending angle of the orbit. Example: The min for End Roll is –120, while the max is 120. The sprites in this segment will end somewhere between –120 and 120 degrees in the direction of the Offset. These values mean that the sprites in this segment will be rotating in two different directions with a spread of 240 degrees. Some sprites that chose an end value in between the min/max values (near 0.0) will barely Orbit, while those at either end of the min/max values will Orbit to a greater degree.