Gentoo Development Guide
Ebuild Functions
When installing packages from source, the function call order is pkg_setup,
src_unpack, src_compile, src_test (optional, FEATURES="test"),
src_install, pkg_preinst, pkg_postinst. When installing packages
from a binary, the function call order is pkg_setup, pkg_preinst,
pkg_postinst.

The pkg_prerm and pkg_postrm functions are called when uninstalling a
package. The pkg_config function is used for any special package
configuration
—
it is only run when explicitly requested by the user. The
pkg_nofetch function is used when a RESTRICT="fetch" package needs to
obtain some SRC_URI components.
Between the transition from pkg_preinst to pkg_postinst, files are
copied over to the live filesystem from the sandboxed temporary installation
location, and Portage records digests of the files installed.
When testing or debugging, you can instruct Portage to execute a specific function
from an ebuild by using the ebuild command, see the ebuild(1) manual
page for further information.