When is a unit "loaded"?

[wonderfly@xxxxxxxxxx (Daniel Wang)]

I have a unit, say foo.service, on my system that's in
/usr/lib/systemd/system, but disabled by preset. On system boot, it doesn't
show as "loaded" per `systemctl --all | grep foo`. So if I override it with
a file with the same name but under /etc/systemd/system, `systemctl cat
foo.service` will show the one under /etc without the need for a `systemctl
daemon-reload`.

If I create another service unit, bar.service, which has a After=
dependency on foo.service, and start bar, foo.service will show as loaded.
And then if I try to override it, `systemctl cat foo.service` will print a
warning saying a daemon-reload is needed.

Nothings seems incorrect, but I have a few questions:
- Which units are loaded on-boot and which are not?
- Is the After= dependency alone enough to have systemd load a unit? Are
there any other dependency directives that will result in the same effect?

-- 
Best,
Daniel
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