What is a 'trait' in unit mod coding?

[Lonecat Nekophrodite]

What exactly is a 'trait' in unit mod coding? Is it a mechanism to activate 'secondary unit class' assigned to a specific unit and why only UUs have 'traits' and not generic unit?
Is 'trait' can also be adapted to adjust unit attributes (combat strenghts for example) that requires a tech to be research or civics to be cultivated?

...
<Row UnitType="UNIT_GREEK_HOPLITE" Cost="65" Maintenance="1" BaseMoves="2" BaseSightRange="2" ZoneOfControl="true" Domain="DOMAIN_LAND"
Combat="25" FormationClass="FORMATION_CLASS_LAND_COMBAT" PromotionClass="PROMOTION_CLASS_ANTI_CAVALRY" AdvisorType="ADVISOR_CONQUEST"
Name="LOC_UNIT_GREEK_HOPLITE_NAME" Description="LOC_UNIT_GREEK_HOPLITE_DESCRIPTION" PurchaseYield="YIELD_GOLD" MandatoryObsoleteTech="TECH_METAL_CASTING"
TraitType="TRAIT_CIVILIZATION_UNIT_GREEK_HOPLITE" PrereqTech="TECH_BRONZE_WORKING"/>
...

an example of a unit that has 'trait'

 

[raen]

The trait is only to add it has a UU. Of course, you can add some unit features to the trait, but for that we usually use UnitAbilities, connecting with classes.