The circle indicates an "Attack Bonus" but this terminology is unfortunately a little confusing.
In a battle each side essentially gets to roll percentage dice for each attack factor they have in the front line. Each "roll" produces a hit, a rout, or nothing. A unit with an AF of 2 gets two rolls as it were, and can score two hits, two routs, one of each, one hit, one rout, or nothing. If such a unit has been doubled to 4 it gets four rolls and you can figure out the possibilities.
Each side does this each round in a battle. It doesn't matter that one side is defending in a tactical sense. Any unit on that side that has a circle around its attack factor gets that bonus.
This bonus increases the chance that a hit will be scored on each "roll" of the dice. For a Pitched Battle the percentages are something like:
Without Attack Bonus: hit 10%, rout 15%.
With Attack Bonus: hit 30%, rout 15%.
In other battles the base percentages are different, but the principle of the bonus works the same way. So the "Attack Bonus" for units with a circled AF are more deadly attacking or defending in that they will score more hits than a unit with the same AF and no bonus.
In city battles the defenders get an advantage but it's expressed in the base percentages. Basically the tougher the city, the easier it is for the defenders to score a hit or a rout. Defenders with an Attack Bonus adjust the percentages the same way as in a Pitched Battle. The only difference is that the base is different.
The percentage for scoring a hit and a rout are different when attacking a city, but the bonus works there in the same way.
Attack bonuses help you in the sense that the faster you wear the enemy down, the sooner the enemy stops hurting you. But it only helps in determining how much punishment you dish out. It never changes how much punishment one side gets from the other side.
A higher defense factor comes into play in absorbing the punishment the other side caused in the current round of combat. If you score 4 hits against Legions, that's two units lost. Four hits against Spanish Infantry will cause four units to be destroyed, obviously. Of course, you decide which units will be lost unless your front line is overwhelmed.
In battles that allow a Command (or Leadership) Bonus, the principles are the same. This Command Bonus increases the likelihood of each AF scoring a hit when it "rolls the percentage dice" as it were.
The manual explains the values for a leadership bonus, but just remember that it's based on the difference between the two leaders. If Hannibal is fighting against a leader with a Command Rating of 4, his leadership bonus will be a full 20% for every unit in the front line. On the other hand a leader with a rating of 7 fighting against a 6 is going to give only 2.5% for each unit in the front line. In that case, by the way, rallying a unit is more important than keeping a "full" bonus of only 2.5%. But sometimes for Hannibal it makes more sense to forgo rallying a unit if he doesn't really need to.
Does this help?
Forced March Games, LLC