The Variation parameter, which is present in all randomizable components, affects the random aspects of these components which cannot be controlled directly, such as positions and intensity of lumps in Perlin Noise, positions of individual bricks in the Bricks component, or the color of gradient stripes in Noise Gradient. The exact numeric value of Variation doesn't matter much: you cannot predict the exact effect that a certain Variation value will have upon the component appearance. You can think of Variation as a way to choose among 30000 predefined random appearances of a component. Components that have the Variation parameter are called randomizable.
Technically, Variation is a random seed which is fed into random number generators inside the component. The same Variation value will always have the same effect on the component appearance, assuming that the values of other parameters stay unchanged.
In addition to the value of the Variation parameter in the component properties ('local' Variation), all randomizable components in a filter are also affected by the value of the Variation slider on the Settings tab in Filter Controls ('global' Variation). Technically, the actual random seed that goes to the component is a sum of the local and global Variation values. This allows to alter the appearance of all randomizable components with a single slider, without the need to adjust the Variation parameter of each component separately.
Some components (Noise Curve and Noise Gradient) have a parameter that can prevent the component from being affected by global Variation. If none of the components in a filter have the Variation parameter, the global Variation slider in Filter Controls is disabled.
