Resouce Affectation - by: antonio.grr

Could you please explain me the exact meaning ad usage of fields
idResourceSelect
idResource
idContact
idUser

Obviously idContact is used when a contact is affected, but I'm not able to understand usage and meaning of other fields.
And what about code

if (SqlList::getNameFromId('Resource', $this->idResource) == $this->idResource) {
$this->idResource = null;
}
in Affectation construct method ?

Thank you for your attention