This info about the Rendering Settings come directly from a forum post by LuucEarth.
I found it incredibly helpful and want to keep it bookmarked for myself as well. Do check out the rest of the thread though for other gems!
Progressive Rendering Tab
Min Update Samples
Controls the minimum number of samples that need to be rendered per render call in the progressive render loop.
The update interval time in seconds is compared to the rendering time since the beginning of this batch render call. This acts as a hint to the renderer. Longer times will generally increase rendering efficiency, while shorter times yield more frequent updates.
Controls the minimum number of samples that need to be rendered before the progressive render loop is allowed to terminate by any of the termination criteria.
The maximum number of samples is compared to the number of samples since the beginning of this progression. The render call will return 1 if the rendering loop is terminated by this termination criterion.
Max Time (secs)
The maximum time in seconds is compared to the rendering time since the beginning of this progression. The render call will return 1 if the rendering loop is terminated by this termination criterion.
Rendering Quality Enabled
The convergence quality estimate is only available in the non-interactive render mode and can, in addition, be enabled and disabled with this attribute. If disabled, rendering will not stop based on the convergence quality and no progress messages will be issued for the current convergence quality.
A convergence estimate for a pixel has to reach a certain threshold before a pixel is considered converged. This attribute is a relative quality factor for this threshold. A higher quality setting asks for better converged pixels, which means a longer rendering time. Render times will change roughly linearly with the given value, i.e., doubling the quality roughly doubles the render time.
Rendering Converged Ratio
If the progressive rendering quality is enabled, this attribute specifies a threshold that controls the stopping criterion for progressive rendering. As soon as the ratio of converged pixels of the entire image is above this given threshold, Iray Photoreal returns the final result for forthcoming render requests. Additionally, the render call will return 1 in this case indicating to the application that further render calls will have no more effect. Note that setting this attribute to a value larger than the default of 0.95 can lead to extremely long render times.
Optimization Tab Settings
Max Path Length
Bounds the maximum number of vertices (bounces) of light paths to contribute to the result. Since this setting cuts off indirect lighting contributions (one example would be the headlight of a car that depends on a lot of indirect effects to look correct), it should only be applied when the rendering has to be accelerated at the expense of physical accuracy.
The Iray Photoreal caustic sampler can be used to improve the quality of caustics in typical turntable scenes. When enabled, Iray Photoreal will augment the default sampler with a dedicated caustic sampler, designed to improve capturing caustic effects. In terms of light path expressions, the caustic sampler was designed to handle paths with the signature E D .* S L much better.
The Iray Photoreal architectural sampler can be used to improve the convergence speed of difficult scenes, as such complicated lighting scenarios can then be handled much more efficiently with this sampler. A common scene type that profits from this specialized sampler is indoor architectural visualization, especially if it is mostly illuminated by indirect lighting. One specific example would be a room that is illuminated by light sources placed in neighboring rooms or by outdoor lighting (such as the Sun and Sky model) shining through a small window.
Filtering Tab Settings
Firefly Filter Enable
Controls a built-in filter to reduce bright spots that may occur under some difficult lighting conditions. Such bright undesired pixels are often called “fireflies”.
The nominal luminance is a hint to Iray Photoreal on what is considered a “reasonable” luminance level when viewing the scene. This luminance level is used internally to tune the firefly filter and error estimate. When the nominal luminance value is set to 0, Iray Photoreal will estimate the nominal luminance value from the tonemapper settings. If a user application applies its own tonemapping without using the built-in tonemappers, it is strongly advised to provide a nominal luminance.
Noise Filter Enable
This specialized post filter reduces initial high variance noise without sacrificing overall sharpness. It is particularly useful for removing isolated bright or very dark pixels that result from certain kinds of disadvantageous chains of scattering events. This filter is intended to be used only during the initial rendering phase, mainly to smoothen out the yet unfinished simulation results. Note that the overall rendering performance can be reduced noticably on low-end CPU cores.
Noise Degrain Filtering
Selects one of several additional post filters that can reduce low frequency noise without sacrificing overall sharpness. These are intended to be used in the final stage of the rendering phase, mainly to reduce remaining subtle grain in difficult areas of a scene. There are five different filters to select from
Modes 1 to 3 are working very conservative and should thus be safe to use in general, modes 4 and 5 are considered to be more agressive and should be used with caution, especially if the scene features fine details in either geometry or applied materials. As these filters can be tweaked during the rendering process, it is recommended to experiment with different radius settings to achieve best results.
Noise Degrain Radius
This value should be reduced if the filter smoothens out edges, and increased if some noise still remains in the image.
Noise Degrain Blur Difference
Modes 4 and 5 feature an additional setting that limits the influence of neighboring pixels if the brightness is too different.