This is a header file, but it can be considered a module due to the large amount of logic that it implements. It defines fcs_state_t (which represents a complete layout of the Freecell board) and of fcs_state_extra_info_t. It contains many macros for manipulating states and cards. (all of them should behave like function calls)
The fcs_state_extra_info_t contains the real positions of the stacks and freecells (refer to (SECTION_REF Canonisation and Normalisation)) and other things that the system uses but don't uniquely identify the state in the state collection.