The KEY directory is an optional output directory. It is given as a subdirectory of DATA and is used to contain a human-readable assembly. The directory's default name is ForDistribution, which is also the name of the module that generates it.
When a draft assembly in a SUBDIR is deemed good enough for submission, it is ready to be packaged in a human-readable form. The module ForDistribution will package the assembly in SUBDIR into such a form, and it will generate also all the auxiliary files needed for submission to NCBI. (See ForDistribution for a list of files generated.) The file source in KEY specifies which SUBDIR the assembly comes from.