I want clarification on the above subject matter. Many authors refer to the two constructs as same. I will also like to to have full discussion on them and other external sources for further details.
In short: usability requirements are the mean to reach appropriate usability attributes for concrete system.
Usability Attributes are some features, which are represent more abstract Usability concept.
Usability has multiple components and is traditionally associated with these five usability attributes:
• Learnability: The system should be easy to learn so that the user can rapidly start getting some work done with the system.
• Efficiency: The system should be efficient to use, so that once the user has learned the system, a high level of productivity is possible.
• Memorability: The system should be easy to remember, so that the casual user is able to return to the system after some period of not having used it, without having to learn everything all over again.
• Errors: The system should have a low error rate, so that users make few errors during the use of the system, and so that if they do make errors they can easily recover from them. Further, catastrophic errors must not occur.
• Satisfaction: The system should be pleasant to use, so that users are subjectively satisfied when using it; they like it.
from Usability Engineering by J. Nielsen
But those are too abstract to use them as non-functional usability requirements for concrete project. So, for example, to claim the system is Efficient, it should be tested against some important points, which in sum give general asessement of the usability attribute.
So Usability Requirements decompose each Usability Attribute on concrete measurable and testable statements having in mind all the elements of the system, like context of use, users, tasks, tools, etc.
Usability requirements document measurable wants and needs for the system and include three type of information:
• The context of use: the intended users, their goals and tasks, associated equipment, and the physical and social environment in which the product can be used.
• Performance and satisfaction criteria: measures of usability for the product.
• The test method and context of testing: the method to be used to test whether the usability requirements have been met and the context in which the measurements will be made.
a usability requirement is what you want the website to do. for instance you want it to be easy to use, attractive, safe, secure, easy to learn etc. A usability attribute is how you can measure each of these requirements. for instance, easy to learn can be measured by the time it takes a user to learn how to complete a particular task on the website. Here, time is the attribute. I hope this helps.