This command reads data into the data structures from external sources. The external sources can be either text files (card images for the ancient among us) or binary files. The fortran unit number from which the information is read, is specified with the unit-spec. Specifying UNIT 5 indicates that the data to be read following the command in the input stream.
The precise format of all these files is described only in the source code as that serves as the only definitive, accurate, and up to date description of these formats. The description of the data structures provides pointers to the subroutines which should be consulted, see Data Structures.