import(P) The library and visible sources of package P are available to satisfy IMPORT requests in the current compilation. The imported sources are not recompiled.
import_version(P, BuildDir) Like import(P), but the library and visible sources of package P are selected from building directory BuildDir rather than the default building directory for the current platform.
include_dir(D) Recursively include the sources named in the m3makefile in subdirectory D of the current directory.
include_pkg(P) Include the sources named in package P's src/m3makefile. The location of P may be overridden.
Import reflects the fact that programs are built from separately compiled pieces, rather than being compiled as a whole. If we always compiled from source, include would suffice.