Metafix is a domain specific language for metadata transformation based on Catmandu FIX. The FIX object performing the transformation is used as part of a processing pipeline.
If you are using Metafacture with CLI or Playground and therefore the Flux scripting language to build and run pipelines, use the fix command in your FLUX-Pipeline.
If you are using Metafacture as a Java library, create a Metafix object and add it to your pipeline (see also the Framework User Guide).
The transformation itself is declared in a fix-object which can be a file. For more information on how to declare transformations see Metafix User Guide.
See here a list for all available FIX functions.
PS: There is also the transformation module MORPH. Have a look at the old documentation and the German cookbook by Swissbib.