Exercise 2: Why a Migration Script Needs Type=oneshot + RemainAfterExit=yes — Possible Solution
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Explanation:

Per the chapter's own compare-table, Type=simple considers a service
"started" the moment its ExecStart= process forks -- and, critically,
systemd expects that process to keep RUNNING as the actual service. A
database migration script is fundamentally different in shape: it's
meant to run once, perform its work, and then EXIT -- there is no
long-running process for systemd to keep tracking as "the service"
after the migration finishes. If this were configured as Type=simple,
the moment the script legitimately finished and exited (having
successfully done its job), systemd would interpret that exit as the
service having stopped/failed, which is a fundamentally wrong reading
of what actually happened -- the script SUCCEEDED, it didn't crash.

Type=oneshot is built exactly for this shape of task -- per the
chapter's own description, it's "a command that runs once and exits,
not a long-running daemon at all." Systemd correctly expects the
process to exit as the normal, successful completion of the unit's
job, rather than treating that exit as a failure needing a restart.

RemainAfterExit=yes is the necessary companion setting because,
without it, systemctl status would report the migration unit as
inactive the instant the script finishes -- even though it succeeded
-- making it hard to tell at a glance whether the migration ever
actually ran successfully. With RemainAfterExit=yes, per the chapter's
own description, the unit continues to be reported as "active" after
the one-time command completes, giving a clear, persistent signal that
the migration has been run.

WHY THIS WORKS AS AN ANSWER
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This explains why Type=simple's own definition of "started" is
fundamentally the wrong model for a run-once-and-exit script, and
separately explains what RemainAfterExit=yes specifically adds on top
of Type=oneshot, rather than treating the two settings as a single,
unexplained combination.