Priority_Filter is an optional read-only property of the IMessageFilterNames interface that returns the filter name expected by the List2 method for filtering by priority.
The Priority property of a message is an integer between 1 and 10 (1 being the most important - "highest priority")that is used by an application to define the order in which a message condition should be handled relative to other messages. Unlike severity, which defines the criticality of a message, priority is intended to indicate ordering of corrective action. Priority is always an optional part of a message as only an application can determine if it is needed and what its value should be.
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.