This is a very good question.
FROM
http://www.hoi4wiki.com/Terrain#Base_terrain_modifiers available in-game or on-line.
Units may have specific modifiers in addition to the base terrain and terrain feature modifiers.
Notes:
- These are as shown in the "Adjusters" section in-game.
- The net modifier for a division is the average of its combat battalions, plus the sum of the modifiers for its support battalions.
- "Attack" and "defense" refer to the attacking and defending sides and affect both offensive (soft attack, hard attack) and defensive (breakthrough, defense) statistics.
- The attack and defense modifiers stack additively with the base terrain value, while the movement modifier is multiplicative with the base terrain movement cost.
- River, amphibious, and fort modifiers stack with that of the terrain type proper.
- Understrength divisions have proportionately smaller bonuses, but penalties are unaffected.
Attack
Note that positive river, amphibious (?), and fort (?) attack modifiers cannot exceed the base penalty. {Table follows}