TdOPCEventFilter represents a single event subscription.
The client creates a subscription (filter) to receive only the desired event notifications from an OPC AE server. Each subscription between a client and server will have one filter only, but several criteria can be included in that filter. Clients can implement multiple filters, each with their own filter criteria. If no filter is established a default filter is created that is equivalent to "no filtering" (all event notifications will be sent). The primary reason for a filter is to reduce unnecessary communication overhead and to improve performance. Therefore the most important filtering criteria are for the severity and the process area. The server logically "ORs" together the list of values for a single filter (e.g. if two event categories are specified, event notifications for both categories will be received). The server logically "ANDs" together specified multiple filters (i.e. only events meeting all criteria will be selected). An example would be to specify both lowest severity and highest severity which then would result in the selection of events with severities lying between the two values. Servers may not support all the various filter criteria. If a filter criterion is specified that is not supported by the server, this filter criterion will be ignored.