Check out the PrerequisiteDef.xsd schema file. You will see these listed as available options
<xs:element name="Unavailable" type="xs:boolean" minOccurs="0" default="false"/>
<xs:element name="Government" type="PrereqOptionGroupDef" minOccurs="0" maxOccurs="unbounded"/>
<xs:element name="Techs" type="PrereqOptionGroupDef" minOccurs="0" maxOccurs="unbounded"/>
<xs:element name="Culture" type="PrereqOptionGroupDef" minOccurs="0" maxOccurs="unbounded"/>
<xs:element name="RaceTrait" type="PrereqOptionGroupDef" minOccurs="0" maxOccurs="unbounded"/>
<xs:element name="TechAge" type="PrereqOptionGroupDef" minOccurs="0" maxOccurs="unbounded"/>
<xs:element name="Stat" type="PrereqOptionGroupDef" minOccurs="0" maxOccurs="unbounded"/>
<xs:element name="Improvement" type="xs:string" minOccurs="0" maxOccurs="unbounded"/>
<xs:element name="StarbaseModule" type="xs:string" minOccurs="0" maxOccurs="unbounded"/>
<xs:element name="ShipComponent" type="xs:string" minOccurs="0" maxOccurs="unbounded"/>
<xs:element name="UpgradesFrom" type="xs:string" minOccurs="0"/>
Some of these like "Techs" & "TechAge" are used by the RaceTechDefs, so I don't know what works with what fully yet.