Part 1 of this Standard describes the Base Language, and all standard-conforming Fortran compilers must implement everything described in Part 1.
Other Parts of the Standard are optional and describe self-contained features which have been requested by a substantial body of users and/or implementors, but which are not deemed to be of sufficient generality for them to be required in all standard-conforming Fortran compilers. However, if a standard-conforming Fortran compiler wishes to provide any of the facilities described in these (optional) Parts of the Standard then they must be provided in accordance with the description of those facilities in the appropriate Part of the Standard.
At present, there are two published Parts of the Fortran Standard:
There used to be a third part:
but this has been withdrawn.