Fortran 2018 is a minor revision of Fortran 2008. Most new features in Fortran 2018 are defined in two Technical Specification (TS) documents:
Current F2018 Working Document as of April 2018: Fortran 2018 Draft International Standard for Ballot (ISO/IEC JTC1/SC22/WG5 N2146)
The new planned features and changes in Fortran 2018 are also summarized by John Reid in a report titled The new features of Fortran 2018 (ISO/IEC JTC1/SC22/WG5 N2145).