External Conversion

I wish to write a conversion program for files containg text in a private format. What are the rules/where can I find them?

[Erik]

To use an external conversion program:
<ul type="square"> Open a pair of the files in the File Viewer.
Select Tools->Edit Current Rules...
Switch to the Conversion tab.
Change the Conversion Type to External Program.
Enter an appropriate command line in Run when loading. For example, C:\MyConverter.exe %s %t (%s is the source filename, and %t is the target filename).
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