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Enabled – Turns the VRayIES light on and off.

Enable viewport shading – When enabled, the effect of the VRayIES light is visible in the viewport.

Show distribution – Shows the IES profile distribution as a mesh around the light source in the viewports.

Targeted – Makes the VRayIES targeted. 

Ies file (button) – Specifies the .ies file that defines the light profile.

Rotation X/Y/Z – Rotates the light shape in relation to the target. 

Cutoff – Specifies a threshold for the light intensity, below which the light is not computed. This can be useful in scenes with many lights, where you want to limit the effect of the lights to some distance around them. Larger values cut away more from the light; lower values make the light range larger. If you specify 0.0, the light is calculated for all surfaces. This parameter is not available when the renderer is set to GPU.

Shadow bias – Moves the shadow toward or away from the shadow-casting object (or objects). If the Bias value is too low, shadows can "leak" through places they shouldn't, produce moire patterns, or make out-of-place dark areas on meshes. If Bias is too high, shadows can "detach" from an object. If the Bias value is too extreme in either direction, shadows might not be rendered at all. 

Cast shadows – When enabled (the default), the light casts shadows. Turn this option off to disable shadow casting for the light. 

Affect diffuse – Determines whether the light is affecting the diffuse properties of the materials. 

Diffuse contribution – A multiplier for controlling the light's contribution to the diffuse illumination.

Affect specular – Determines whether the light is affecting the specular part of the materials. 

Specular contribution – A multiplier for controlling the light's contribution to the specular reflection.

Affect atmospherics - When enabled, the light influences the atmospheric effects in the scene.

Atmospherics contribution - Determines the amount of influence the light has on the atmospheric effects.

Use light shape – Determines whether and how the shape of the light is taken into consideration during the calculation of shadows and illumination. 

No – The light shape is not considered for illumination and the light is going to generate sharp shadows. 
For shadows – The light shape is considered only for the generation of soft shadows. The illumination is not affected by the light shape (i.e. it behaves as a point light). 
For illumination and shadows – The light shape is considered for the calculation of the illumination and shadows.

Override shape – When enabled, the light profile within the .ies file is ignored. The Shape parameter defines light shape for calculating shadow softness. Sharp shadows are generated with the point type.

Shape – Specifies a shape (and its orientation) for the VRayIES light when Override shape is enabled. Some shapes (shown below) enable additional options for size. See the Shape example below.

Height – Specifies the height for applicable light Shape types.

Width – Specifies the width for applicable light Shape types.

Length – Specifies the length for applicable light Shape types.

Diameter – Specifies the diameter for applicable light Shape types.

Color mode – Specifies the mode in which the color of the light is determined:

Color – The light color is directly specified by the Color parameter. 
Temperature – The light color temperature (in Kelvin) is specified by the Temperature parameter.

Color – Specifies the color of the light.

Color temperature – Specifies the light color temperature in Kelvin.

Override intensity – Allows you to choose how the IES light source intensity is overridden by the Intensity value parameter. 

Rescale – The IES file's default global light intensity is multiplied, so it matches the user's defined light intensity.
Replace – The IES file's default global light intensity is recalclulated from the light source distribution and then multiplied to match the user's defined light intensity. 

Intensity type – Specifies the units for light strength:

Power (lm) – The Intensity value is considered in lumens.
Intensity (cd) – The Intensity value is considered in candela

Intensity value – Specifies the strength of the light.

Area speculars – When disabled, the particular light is rendered as a point light in the specular reflections

Viewport wire color – Changes the wireframe color of the light in the viewport. This is useful to visually recognize groups of lights from one another.

Icon text – Turns the viewport icon text on or off.

Decay near on – Toggles near decay on and off. The Decay parameters determine how the light fades in and out. The Decay Near determines how light "fades in". The light isn't at its maximum value at its source, but instead gradually increases until it reaches the Decay near end. The Decay Far determines how light "fades out". The light isn't at its maximum value at its end, but instead gradually decreases after the Decay far start. See the Near examples below for more information.

Decay near start – Determines where the "fade in" starts. If set at 0, light won't be visible directly in front of the light source, but will instead "fade in" from it.

Decay near end – Determines where the "fade in" ends. After this threshold, the light is at its full value.

Decay far on – Toggles far decay on and off. See the Far examples below for more information.

Decay far start – Determines where the "fade off" starts.

Decay far end – Determines where the light reaches a value of 0, i.e. completely fades off.

Exclude... – This button brings up the include/exclude list which allows users to exclude objects from light's illumination and shadow casting.