If it is not one of the pre defined Properties in this manual http://pubs.vmware.com/vCAC-60/topic/com.vmware.ICbase/PDF/vcloud-automation-center-60-custom-property-reference.pdf
then it should be defined in the Property Dictionary under Infrastructure/Blueprints/Property Dictionary. If it is not defined there then it could be created in a build profile. What this custom property could be referencing though might actually be reference for a vCO workflow so may be defined in that workflow.