From LuxRender Wiki
The LuxBlend system tab contains a variety of variables that do not easily fit in any of the other tabs.
This section should contain the paths to the folder in which the LuxRender executable file is located. This file is called luxrender.exe (for Windows), luxrender (for Linux) or luxrender.app (for OS X).
Additionally, a folder should be chosen in which LuxRender's scene files and output images should be saved.
This setting defines how many process threads LuxRender should use. When using Auto Detect, the program will use as many threads as there are processor cores available as this gives the maximum rendering speed.
Alternatively, the number of threads can be set manually. During the rendering process, the number of threads to be used can still be changed in the graphical interface.
Various LuxRender parameters can be animated using IPO curves, including camera and environment parameters and render settings. By pressing the "Enable Parameter IPO Keyframing" button, small buttons with an "I" on them will appear on the right of a number of variables in LuxBlend. By pressing any of those buttons, the name of the Blender IPO curve to be used for animating the parameter can be entered.
This button is not functional yet.
This sets the default quality for material previews in the material tab.
Reverse gamma correction (RGC)
When using photographs, scanned images or other images that are originally intended to be displayed on a computer monitor as image textures, the reverse gamma correction compensates for the fact that brightness curves on a monitor are not linear. In other words, it converts images that have a gamma of 2.2 to a gamma of 1 before applying them as textures. This usually gives a better result.
This limits the brightness of materials to 90%, so that even the brightest objects in the scene will absorb some light (as is usually the case in reality). If very bright (almost white) materials are present it will tend to create a lot of fireflies in your image and slow your rendering speed. Switching colour clamp on will result in a slight increase in render speed in this case.
However, this button is not enabled by default because it clamps all colors, even those used with glass materials resulting in dark glasses. You should only switch CoClamp on if you don't have any glass in your scene.
By selecting "optimize mesh", the scene geometry will be exported in a way that is optimised for LuxRender. This does not cause any visible changes in the geometry.
If instanced objects are present in your scene, they can be defined as instances in LuxRender as well. The treshold value defines whether or not this will happen, based on the number of times the instanced object is present in the scene. Using instances decreases LuxRender's memory usage and scene file size while it does not typically decrease render speed.
This will convert all Blender materials in the scene into LuxRender materials, based on a number of basic settings. It will overwrite settings that have been manually changed in LuxBlend's material tab.
This will save the current LuxBlend settings as default, so that the next time LuxBlend is started the same settings will be used.