An architecture tactic is a specific refactoring of a portion of the architecture. The tactic provides specific benefits in terms of enhanced levels of specific quality attributes.

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.

Here are resources on tactics:

Security and Survivabiity reasoning frameworks 

Deriving Architectural Tactics

Modifiability tactics

Realizing and Refining Architectural Tactics: Availability