Interpolation Controllers (ICs) enable you to vary the parameters by particle age (default PA controller), particle velocity, distance from the emitter, distance from any other object in the scene, or by using expressions. The easiest way to visualize how each controller works is to enable the "Show in Viewport" button in the Particle Source User Interface and to enable the Radius Controller.
In order to change between the various Interpolation Controller types, right - click the Controller icon. You'll get the following dialog.
When you click the left mouse button over the Interpolation Controller, its dialog will show up. Not all Interpolation Controllers have user interfaces.
Each controller is described below.
This is the default controller and it has no user interface. The ramp that uses this controller will interpolate between Low and High values based on the particle's age.
The Interpolation between Low and High values is based on the particle velocity. Interpolation is calculated using the ramp only if the particle velocity is in the Minimum to Maximum velocity range.
The interpolation between Low and High values is based on the particle distance from the particle system emitter. If the particle is further away than specified by the Maximum parameter only the High value will be applied. If the particle is closer than specified by Minimum, the Low values will be applied. All other distances ranging from Minimum to Maximum will be used to interpolate the parameter.
This Controller is similar to the Emitter Distance Controller, but instead of using a particle emitter as the only reference object, you can attach any object from the scene.