of the custom attribute definition and its corresponding custom attributes. This value
can be a simple key, which is the key that is provided when the custom attribute definition
is created, or a qualified key, if the requesting
application is not the definition owner. The qualified key consists of the application ID
of the custom attribute definition owner
followed by the simple key that was provided when the definition was created. It has the
format application_id:simple key.
The value for a simple key can contain up to 60 alphanumeric characters, periods (.),
underscores (_), and hyphens (-).
This field can not be changed
after the custom attribute definition is created. This field is required when creating
a definition and must be unique per application, seller, and resource type.