A light source is the part of a lighting fixture that emits light (such as a light bulb).
In general, each lighting fixture family has one light source. To create a lighting fixture that uses multiple light sources (such as a chandelier or a set of track lights), create a nested family.
When a light source is selected in the Family Category and Parameters dialog, you can specify the shape of the light element (point, line, rectangle, circle), and the light distribution (spherical, hemispherical, spot, or photometric web). You can also define photometric characteristics, such as Light Loss Factor, Initial Intensity, and Initial Color Control. In a project, you can adjust the position and brightness of each light source to achieve the desired lighting effects.
When creating a lighting family, you can also specify an IES file. This file contains engineering data that can be used to calculate the coefficient of utilization of the fixture. The IES file is not used for rendering. Lighting manufacturers often allow you to download IES files from the Web for their fixtures. Some IES files are provided here:
%ALLUSERSPROFILE%\Autodesk\RVT 2018\IES