Metafile

From Fmepedia

A metafile contains directives that inform FME of the details about a format such as default parameters, settings, etc.

Creation or editing of metafiles is generally not required in standard FME use - which is why I'm not going to mention where they can be found - they are of more relevance when using the FME plugin SDK to create your own format.

For users of the FME Plugin-SDK there is a specific metadata reference page.