ProcessHost_Filter is a read-only property of the IMessageFilterNames interface that returns the filter name expected by the List2 method for filtering by the fully qualified domain name (FQDN) of the machine that sent the message.
The MessageLog2.SendLogMessage method will automatically add the ProcessHost to any log message sent to a PI server's Message log (remote messages). When sending messages to the local log it is assumed the process host is the same as the host of the PI Message subsystem and this parameter is not logged.
The object placeholder is an object expression that evaluates to a IMessageFilterNames interface.
The IMessageFilterNames interface can be obtained directly from the MessageLog2.MessageFilterNames property or by using QueryInterface (assignment, in VB, casting in .Net) on a MessageLog or MessageLog2 object.
When adding to the NamedValues collection, the IMessageFilterNames interface acts with Intellisense to show the available parameter options and ensures only names that are supported are used.