two questions :
1) When I'm in the shop when I check the weapons there's an "active" slot with a number and a red arrow down, what does that mean?
2) How damage is calculated? Sometimes it seems to be attack - defense, but sometimes it's more than that. It's the bonus from a "green arrow up" weapon? How much is that bonus?
1) That's the damage diference between the current equiped weapon, and the weapon you have selected on the store. In your case, you would be losing 6 damage by downgrading to the item you are selecting
2)It is attack - defense, kind of. Terrain may affect the defense.
About the arrow up: Pretty sure that is the weapon triangle.
Swords > Axe > Lance > Sword
The character that has the advantage gets +1 damage , + 10 Hit%
The other character gets -1 damage, - 10 % hit.
If you are planing to play Fates next, you should know the weapon triangle and a lot of the weapon mechanics were changed.
As a side note, some weapons have an advantage over a class, like Bows vs Flyers, in this case, the base damage of the weapon is tripled before calculations. That is why Archers can OHKO Pegasus Knights.
Wind magic its suposed to be good against Flyers too, but the very first tome has only 1 Attack, making it kind of underwhelming for a counter weapon.
