"What exactly is tactical attack?"
In simplest terms, tactical attack would be "blowing up" enemy units.
Strategic attack would be "starving enemy units" to death. In broader terms - strategic attack would also include starving the enemies ability to create new units, either by blowing up the factories or starving the factories of resources necessary for unit production.
Tactical attack is attacking combat units: infantry/tanks/artillery/destroyers-battleships.
Strategic attack is attacking enemy means to create or supply units:
Strategic attack on creating units would be an attack primarily at industry. although attacking infrastructure and resource convoys which supply industry is also arguably a strategic form of attack.
Strategic attack on means to supply units would be attack on infrastructure carrying supplies to front line units, and attacking supply convoys carrying supplies to enemy ports (which "feed" the combat forces supplied by the port)
Lastly, you could argue that encirclement is a form of strategic attack since its objective is to destroy enemy means to wage war (units) by the method of "starving" enemy units of fuel and supplies - as opposed to destroying the enemy units by "direct contact" forms of attack (tactical attacks) such as superior air/ground/naval attacks.
So a bonus to tactical air attack means greater force when hitting units.
A bonus to strategic air attack would mean greater force when hitting infrastructure - factories - ports - air fields etc.