Backup
A backup, in the way most ledger platforms use the word, is a copy the
platform holds and manages inside its own systems. It exists to
protect the platform's own infrastructure against its own failures.
Reading it, if it can be read at all, means signing in to the same
platform whose failure you might be trying to recover from.
Export
An export is a file someone deliberately pulled out of the source
platform at a specific moment: a CSV, a PDF, a downloaded report. It
is accurate for whatever it captured and immediately stale for
everything that changed afterward. Its scope is whatever the export
screen included that day, which is easy to under-scope without
noticing.
Independent copy
An independent copy is written to storage controlled by a party other
than the source platform, on a recurring, dated basis. Its value comes
from where it is held: because it sits outside the platform that
produced it, its contents can be checked even if that platform is
unavailable, misconfigured, or wrong about its own state.