CREDENTIALS_DIRECTORY vs %d

[Gesh <gesh@xxxxxxxxxxx>]

systemd.exec(5) reads:
> In order to reference the path a credential may be read from within a
> ExecStart= command line use "${CREDENTIALS_DIRECTORY}/mycred", e.g.
> "ExecStart=cat ${CREDENTIALS_DIRECTORY}/mycred". In order to reference the
> path a credential may be read from within a Environment= line use "%d/mycred",
> e.g.  "Environment=MYCREDPATH=%d/mycred".

Is there a reason that the `ExecStart=` line should specifically refer to
`$CREDENTIALS_DIRECTORY` and not `%d`? Surely these are equivalent?

Thanks,
Gesh