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Here is a rendered image along with a heat map of the sample counts to show this. From left-to-right, the three teapots in this image have their relative pixel variance set to 20, 1, and 0, but are otherwise identical. The scene's pixel variance is intentionally set somewhat high to emphasize the difference in noise:
|Before After Image Slider|
There are some limitations to this, however. If the maximum samples are not set high enough, then lowering the relative pixel variance on undersampled objects may not help much. Another limitation is that this only works on objects directly visible to camera rays; it does not affect the sampling of the shadow cast by the object, nor does it affect the sampling when seen indirectly through reflection or refraction. For these, you would need to adjust the relative pixel variance on the object receiving shadow, or doing the reflection or refraction. Finally, note that an extreme difference in sampling between an object and the background behind it can produce a halo-like effect.
Adaptive Sampling Tips
For more help on image quality, see the documentation on Reducing Noise.
Variance, or noise, receives more samples than surrounding areas with less variance.