The disk light shape simulates light from a circular area source (flat disk).
The lights page has more detail about the controls. The additional parameters are shown below. In addition to honoring the standard 3ds Max light parameters, the Modify Panel will also show the following parameters:
Disc light parameters
The radius of the disk. Disk lights will always be circular. It is not possible to scale the width or height to create an ellipse. When rendering, Arnold stores a disk light as just a point and a radius. Any transforms are applied to the position, but not the radius.
Emits light focused in the direction along the normal. The default spread value of 1 gives diffuse emission, while lower values focus the light more until it becomes almost a laser-like beam at value 0. Currently fully focused laser beams at value 0 are not supported, there is always a small minimum spread. Low spread values can be noisier than the default high spread, so be careful when using them.
The animation below shows the effect when lowering the Spread value.
Spread value lowered from 1 (Quad light)
Further example of a scene lit with a disc light