Requests or impressions that do not come from a real person on a real device: bots, datacenters, spoofed apps.
General IVT is caught with lists and simple rules, such as datacenter IP ranges. Sophisticated IVT needs a specialist vendor. Filtering at the request is cheaper than refunding at the invoice.
General IVT is caught with lists and simple rules at the request. Sophisticated IVT needs a vendor. Filtering early is cheaper than refunding later.