Mod boxes are one of the rare items in wooden chests. When you open one of these boxes you will will receive one of the modded items available from the pool based on the researches you have already unlocked with your knowledge stars.
The items that can drop in order from common to rare are:
-Drawers: Storage upgrade template, Void Upgrade, Iron Storage Upgrade, Gold Storage Upgrade, Diamond Storage Upgrade, Obsidian Storage Upgrade, Framed Compacting Drawer, Framed Drawer controller, Emerald Storage Upgrade
-Refined Storage
-Applied Energistics
-Powah
-Therman Dynamos
-Thermal Expansion
-Mekanism Generators
-Create
-Mekanism
-Botania
-Easy Villagers