Custom AOVs can be created through specific node types. The node will define the AOV type, and the node's name defines the AOV name. The node types related to custom AOVs are grouped in the AOV category of 3Delight shading nodes in the Hypershade.
The various 3Delight shading node types related to custom AOVs.
Custom AOV nodes are presented in the AOV Selector's Custom section, and can be selected for output to an image layer just like the built-in AOVs.
Float and Color Custom AOVs
AOV Group