Boosts are applied to items via Deltamod stats, in combination with the values in Shared's ItemTypes stats.
For instance, here's the Uncommon entry in ItemTypes:
Notice the entries for "BaseUncommon".
Now in DeltaModifier, you'll see values like these:
This is what actually applies boosts to items. In this case, when an Uncommon rarity item is created, it has chances to roll boosts for all the entries in ItemTypes, and then the actual boosts are determined via Deltamods. So if a cloth type helmet gets a "BaseUncommon" deltamod, it receives "_Boost_Armor_All_Armour_Physical_Base" and so forth.
Deltamods of the BoostType "ItemCombo" aren't rolled on by the system, but are for applying deltamods with crafting, or possibly scripting.