Custom attributes are attributes you optionally add and define from the Add Attribute window. Although custom attributes are dynamically added to an object, we refer to them as custom to distinguish them from the built-in dynamic attributes.
Custom attributes have no direct effect on any characteristic of an object in Maya. You can use them to control a combination of other attributes. You might also use a custom attribute as a variable—a place to store a value temporarily to be read by other attributes.
When you add a custom attribute to an object, it appears in the Extra Attributes section of the Attribute Editor (and in the Channel Box, if you make the attribute keyable).
Turn on to create a corresponding initial state attribute for the added attribute. Without this corresponding attribute, you can’t save a particle object’s current attribute values for initial state usage. You must write a creation expression if you decide to initialize the custom attribute’s value upon rewinding the animation. If you know you’re going to write a creation expression for a custom attribute, you can set Add Initial State Attribute to off when you add the attribute. Otherwise, set Add Initial State Attribute to on whenever you add a custom per particle attribute.
When you're adding a new Enum attribute, you need to define the list of acceptable strings. There are two default strings, “Green” and “Blue”, in the Enum Names list that you can change. To change, select Green or Blue and then enter the new string in the New Name text box. To add a new string, click the blank entry below the last list item and type the string in the New Name text box.
The following list of names are reserved internally for (dynamic) shading attributes. You may use these names for custom attributes, but beware they may produce unexpected results in the shading network. For example, during shading the uvCoord value is provided, therefore ignoring your custom uvCoord attribute value.
