The target directory ❶
Filename and Subdirectories
The two templates ❷ define the directory/files structure that will be created. The field values of each specific message configures which file the message will be written to and (optionally) in which subdirectories the file is located.
The File Contents ❸ section converts messages to text. You can add ❹ as many message formats (parts) as you need.
Also, each part contains a filter expression that defines for which messages this format is applicable. For most use cases one Messages format with blank filter ("any message") is enough.
Q: Why do I need more than one message format?
A: Different syslog message types should be converted to text in different ways.