A quality attribute is a property of a product.

Dilbert on quality attributes

A non-functional requirement sets the required level of a specific quality attribute for a specific product.

The value desired in the product leads us to make certain tactical decisions about the design of the architecture.

An Architecture tactic can be viewed as:

stimulus -> tactical design decision -> estimated response

Usability is a quality attribute.

Tactics such as Undo enhance the level of usability for a product that includes that feature.