Typically cylinder lights are used to simulate fluorescent tube lighting
The cylinder light shape simulates light from a cylindrical area source (tube shape).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:
Cylinder light parameters
Increasing the size of the cylinder light will create a larger area light size and therefore will soften the shadows that run perpendicular to the cylinder’s axis. Cylinder lights will always be circular. It is not possible to scale the width to create an ellipse.
Scales the height of the light.
Further examples of cylinder lights