Dynamic package loading is a new feature in v2.15 that allows for package directories created in the iraf$extern directory to be automatically defined when the CL is started. The means that external package installation no longer requires that the hlib$extern.pkg file be edited to define the package, although that remains an option for packages which somehow cannot conform to this new scheme.
The option to load external packages from the IRAF repository is removed, since the repository is not available anymore.