Fortran 2003 was a somewhat larger revision to Fortran than Fortran 95. Some additions made in Fortran 95, changes in the handling of ALLOCATABLE arrays, and IEEE 754 floating-point exception handling, were taken further.
Many of the added features in Fortran 2003 are somewhat on the esoteric side. One, known as parametrized defined types, however, can be explained.
It was noted in the discussion of FORTRAN IV that the version of FORTRAN IV for the IBM System/360 included, as an extension, a version of the COMPLEX data type in which the real and imaginary parts were DOUBLE PRECISION instead of REAL.
The parametrized defined types feature simply allows user-defined data structure types to have parts that can be of different sizes in different versions of the same type.