[Lord Magus]

I assume you mean "attribute bonus" when you talk about the attribute modifier. The attribute bonus corresponding to the base attribute (base score+bonuses at creation (eg racial) +permanent bonuses gained during play) is stored in the aBonus field for each of the 6 ability scores.

The final attribute bonus, including temporary modifiers to the attribute score, is stored in aModBonus.The final values are available in the Post-Attribute phase, so they should be modified earlier than that if you want to make sure your modifiers are taken into account.

I am not sure at which timing those 2 particular fields are finalized (somewhere in the Attributes phase), and if the way to determine their result is a fixed calculation (which would cancel bonuses specifically granted to those fields earlier).