In CoreAbilities.xml, line 9419 we have

           <GameModifier>
                <ModType>City</ModType>
                <Attribute>TrainingBonus_ApplyAbilityBonus</Attribute>
                <StrVal>Constitution</StrVal>
                <Multiplier>1.5</Multiplier>
                <Provides>Units trained here gain the Constitution trait</Provides>
            </GameModifier>
 

This is part of the ability bonus from killing Morian and obtaining the Heart of Morian.

1) All other Game Modifiers with attribute TrainingBonus_ApplyAbilityBonus have <ModType>CityTrainingBonus</ModType>

eg for Fist of Vetrar we have

           <GameModifier>
                <ModType>CityTrainingBonus</ModType>
                <Attribute>TrainingBonus_ApplyAbilityBonus</Attribute>
                <StrVal>ResistCold</StrVal>
                <Provides>Units trained here gain the Cold Resistance trait</Provides>
            </GameModifier>
 

or the Watchtower improvement has

       <GameModifier>
            <ModType>CityTrainingBonus</ModType>
            <Attribute>TrainingBonus_ApplyAbilityBonus</Attribute>
            <StrVal>Charge</StrVal>
            <Provides>Trained units get +1 Move and +2 Attack on the first action in combat</Provides>
        </GameModifier>

2) The intent seems to be to give the trait 'Constitution' to new trained units. Constitution seems to give 3 extra hit points.

   <AbilityBonus InternalName="ConstitutionAbility">
        <AbilityBonusType>Unit_Design</AbilityBonusType>
        <AbilityBonusOption InternalName="Constitution">
            <DisplayName>Constitution</DisplayName>
            <Description>+3 Hit Points</Description>
            <Icon>Ability_Constitution_Icon.png</Icon>
            <GameModifier>
                <ModType>Unit</ModType>
                <Attribute>AdjustUnitStat</Attribute>
                <StrVal>UnitStat_HitPoints</StrVal>
                <Value>3</Value>
            </GameModifier>
            <AdditionalTrainingTurns>10</AdditionalTrainingTurns>
            <Type>Defensive</Type>
            <AIData AIPersonality="AI_General">
                <AIPriority>5</AIPriority>
            </AIData>
        </AbilityBonusOption>
    </AbilityBonus>

However there is another parameter in the GameModifier XML, <Multiplier> which seems to want to multiply hit points by 1.5 according to the description of the Heart of Morian:-

 <Description>The heart of Morian, Ruin of Summer.  Deep within his corrupted shell was a heart that still remembered verdant forests and unscarred lands.  (+50% growth, +50% hit points for units trained in this city)</Description>

And there is a unit stat called "UnitStat_Constitution" which might be used for this purpose.

   <UnitStatType InternalName="UnitStat_Constitution">
        <DisplayName>Constitution</DisplayName>
        <DisplayNameShort>CON</DisplayNameShort>
        <Description>Determines hit points.</Description>
        <Icon>Stat_Constitution_Icon.png</Icon>
        <Hidden>1</Hidden>
        <BaseSovereignAttribute>1</BaseSovereignAttribute>
        <DefaultValue>6.0</DefaultValue>
        <UnitStatGrouping>AbilityStat</UnitStatGrouping>
        <UnitStatGrouping>UnitDetailsAttribute</UnitStatGrouping>
    </UnitStatType>

Does anyone know what "Heart of Morian" is actually doing? It looks a bit confused to me.