But what I want to know is what are the other modifiers...

When say a suit of armour says

Slashing 2
Crushing -1

What does the positive 2 on slashing do in comparison to the negative value for crushing?

If they were both negative values I'd reckon they reduced the damage from those types of weapons, but because one is positive it makes no sense. And I certainly don't believe that wearing certain armour types makes you take MORE damage from different weapon types (e.g. extra 2 damage if hit by slashing weapons, but -1 from crushing)