UIPI is a security feature that prevents messages from being received from a lower integrity level sender. All such messages with a value above WM_USER are blocked by default. The filter, somewhat contrary to intuition, is a list of messages that are allowed through. Therefore, adding a message to the filter allows that message to be received from a lower integrity sender, while removing a message blocks that message from being received.
Certain messages with a value less than WM_USER are required to pass through the filter regardless of the filter setting. You can call this function to remove one of those messages from the filter and it will return TRUE. However, the message will still be received by the calling process.
Processes at or below SECURITY_MANDATORY_LOW_RID are not allowed to change the filter. If those processes call this function, it will fail.
