For example:
In the parameters dialog of the writer-plugin a file was chosen from the hard disc and the filename is availiable in the parameters list.
Now if we add the FeatureTypeImporter in the FME via Writers - Import Feature Types ... and select in the Format: field the FeatureTypeImporter we want to have this file displayed in the Dataset: field.
The Dataset field is defined in the fmf file of the FeatureTypeImporter and a default value is set there.
There also exists a parameter in the writer fmf file for this file.