The FS (from "feature structure") file format is older, simpler and less expressive one of the two PDT file formats. It is not intended to be the main format (which is the SGML-based CSTS) but it cannot be disposed of because some important tools (e.g. the tree viewers) rely on it. Its description has two parts. The general part explains the overall syntax, how attributes of nodes are defined and how the nodes form a tree. The PDT-specific part gives an overview of the node attributes defined for PDT FS files.
Besides the two parts of the FS description, you may also jump to the description of the CSTS format (from "czech sentence tree structures"), and to the convertor between the two.