I am trying to message (email or sms) only clients for a specific Session Type who have not yet placed an order. In order to use the results of the filter to send messages, I need the base filter to be Clients. But in order to achieve my end goal, I think I need to further refine the Clients by those with a Session of a certain Session Type. I realize I can use a Client Group to approximate Session Type, but I still need to figure out from that Client Group or Session Type attendee who has placed an order, and to do that without manual intervention, I have to address Invoices / Orders. I can't do that as stands.