In the destination node you can use a number of options to name your file. If you click on the Maginifying Glass it will open a Node Tag lookup to assist.

Options Available:
ChannelCache - Any value stored in the channelCache on the source node.
Date/TIme - This is used to select a standard date/time format for the System Node Tag of System_Date
MessageCache - Any value stored in messageCache while processing the message.
Message - Any value in the message. Example {PID-3} gets you PID-3 from an hl7 message.
Souce - Any value in the source(Read-Only copy of what was receieved before the mappings). Example {PID-3} gets you PID-3 from an hl7 message.
System Node Tag - Examples: Endpoint_URL, System_Date, UTC_Date, UUID, UUID-NO-DASHES
System Variable - Any value stored in the System Variables.
Web Service Password - Password stored on a Web Service Connection
Web Service Username - Username stored on a Web Service Connection