The Compound Blur effect blurs pixels in the effect layer based on the luminance values of a control layer, also known as a blur layer or blurring map. By default, bright values in the blur layer correspond to more blurring of the effect layer, while dark values correspond to less blurring; select Invert Blur for light values that correspond to less blurring.
This effect is useful for simulating smudges and fingerprints, or changes in visibility caused by atmospheric conditions such as smoke or heat, especially with animated blur layers such as those generated by the Turbulent Noise effect.
Control Layers Used
Maximum Blur: The maximum amount, in pixels, that any part of the affected layer can be blurred.
Stretch Map To Fit: Stretches the control layer to the dimensions of the layer to which it is applied; otherwise, the control layer is centered on the effect layer.
Steve Holmes provides a video tutorial on the Artbeats website that demonstrates the use of the Compound Blur effect.
(From Adobe Help Site) https://helpx.adobe.com/after-effects/using/blur-sharpen-effects.html#compound_blur_effect