A shader family is a way of categorizing shaders so that users know (usually indicated in the UI by color) how they can be used. When you use one of the Softimage families for your custom shader definition, the same restrictions apply as for native Softimage shader definitions. For example, if you make the mrLight the primary family for your custom shader definition, Softimage will only allow users to connect it to a light in the render tree.
You can use one of the or . You can specify more than one family on a custom shader definition, but you must specify at least one, or else when your shader definition is instantiated in Softimage it will fail. If you specify more than one family for your shader definition, one of them is considered to be primary, and it is that family that determines what restrictions apply as well as the color of the shader node in the render tree.