Individaul Ubnits are assigned to Ability Classes in table "TypeTags" Code: <TypeTags> <Row Type="UNIT_EGYPTIAN_CHARIOT_ARCHER" Tag="CLASS_LIGHT_CHARIOT"/> <Row Type="UNIT_SUMERIAN_WAR_CART" Tag="CLASS_HEAVY_CHARIOT"/> <Row Type="UNIT_HEAVY_CHARIOT" Tag="CLASS_HEAVY_CHARIOT"/> <Row Type="UNIT_TANK" Tag="CLASS_HEAVY_CHARIOT"/> <Row Type="UNIT_MODERN_ARMOR" Tag="CLASS_HEAVY_CHARIOT"/> </TypeTags> Any unit assigned to an individual Ability Class will share in the effect of a UnitAbility that is assigned to be effective for that class of unit: Code: <TypeTags> <Row Type="ABILITY_HEAVY_CHARIOT" Tag="CLASS_HEAVY_CHARIOT"/> <Row Type="ABILITY_LIGHT_CHARIOT" Tag="CLASS_LIGHT_CHARIOT"/> </TypeTags> An individual unit can be assigned to multiple different Ability-Classes, and will share in the UnitAbility effects of any Unit Ability which is thus made valid for that Class of "Ability" UnitAbilities (and their modifiers) are not directly tied to individual units -- they are tied to Ability-Classes, which in turn have individual units assigned to them. The Ability-Class (ie, "ABILITY_CLASS" in the "Tags" table) is used as a convenient catch-all for multiple units when desired without needing to individually code an Ability as being valid for a specific unit or combination of units anywhere else in the database. Individual units can and often are by Firaxis assigned to their own special Ability-Class. So far as I know (at least in the Vanilla expansion) the only unit that was assigned to the "CLASS_LIGHT_CHARIOT" was the Egyptian unique chariot-archer unit. In this way unique or special units can have their own "private" set of abilities without needing to add a bunch of extra single-case Boolean type stuff as was so often done in Civ5 with Promotions and the like.  Without directly looking at which modifiers are attached to a given UnitAbility, and what those individual modifiers actually do, there's no quick and easy answer as to what the difference between two discreet Ability-Classes are. The Light Chariot Ability is an extra 2 movement when starting a turn on "clear" terrain as I recall, but I don't remember what the Heavy Chariot ability is off the top of my head. [edit edit] A quick check of the UnitAbilities.xml file shows that the Heavy Chariot ability is to only give +1 movement on "clear" terrain, so essentially the difference in the two Abilities / Modifiers gives the Egyptian Chariot-Archer an extra extra move in the proper conditions.