Pixel clamping can be useful in scenes that contain 'spike noise' or 'fireflies': isolated, super bright pixels that jump around from frame to frame.
With this control enabled the pixel samples of the AOVs will also be clamped. AOV clamping will affect every RGB and RGBA (except the A component) AOV. Currently, there is no way to tell Arnold which AOV's to clamp and which ones to preserve.
If enabled, this control will clamp pixel samples to this specified maximum value. This can make it easier to anti-alias certain high-dynamic range effects such as bright motion blurred specular streaks (at the cost of reduced contrast). What is clamped is the resulting output value of the renderer computations of each pixel sample. Example: If the Max Clamp Value is set to 2, no pixels will have an RGB value greater than 2.0, 2.0, 2.0
The threshold to clamp away fireflies from indirect light samples and reduce noise. This works similar to AA_sample_clamp but preserves specular highlights from direct lighting. Lower values result in more aggressive noise reduction, possibly at the expense of dynamic range.
This can also help with diffuse scenes when a light is very close to the ceiling/walls.