FDF stands for Flexible Data Format, designed within the Siesta project to simplify the handling of input options. It is based on a keyword/value paradigm (including physical units when relevant), supplemented by blocks for arbitrarily complex blobs of data and lists.
LibFDF is the official implementation of the FDF specifications for use in client codes. At present the FDF format is used extensively by Siesta, and it has been an inspiration for several other code-specific input formats.
New input options can be implemented very easily. When a keyword is not present in the FDF file the corresponding program variable is assigned a pre-programmed default value. This enables programmers of client codes to insert new input statements anywhere in the code, without taking care of ``reserving a slot'' in a possibly already crowded fixed-format input file.