File Formats

I just heard about rdiff-backup and I'm planning how to configure it.

The documentation says:
"Earlier states of your files are saved just by 1) keeping a copy of them,
2) in diff form as produced by rdiff, or 3) as a gzipped version of 1 or 2."

I see the --no-compression option to disable compression. However I do not
see an option to produce copies of older files vs. storing them in rdiff
format. How is the file format for older files controlled?

Thanks much!