S/T Mode -- Backup breathing rate -- Advanced!!

Any techies know how the backup breathing rate is determined when a
Doctor writes a prescription? I am interested in learning how a clinician
determines if a patient actually needs a Timed Backup with his BiPap machine.
If after using a BiPap with the Timed Backup rate feature turned on, and
one sees in the AutoScan report that the Percentage of Spontaneous
breaths was 45%, then it should be pretty clear that the Backup feature is
needed. But what if after a couple of weeks the Percentage of Spontaneous breaths was over 95%. Is this evidence that a Timed Backup Rate feature
is probably not needed? Thanks.