Attribute is used in several contexts, I suspect Robin Asby is using it in the philosophical frame.
In logic and philosophy (especially metaphysics), a property is a **characteristic of an object**; a red object is said to have the property of redness. The property may be considered **a form** of object in its own right, able to possess other properties. A property, however, differs from individual objects in that it may be instantiated, and often in more than one object. It differs from the logical/mathematical concept of class by not having any concept of extensionality, and from the philosophical concept of class in that a property is considered to be **distinct from the objects** which possess it. Understanding how different individual entities (or particulars) can in some sense have some of the same properties is the basis of the **problem of universals**.
https://en.m.wikipedia.org/wiki/Property_(philosophy)
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Wikipedia 