Standby, reserve or extra.
Supporting, reinforcing; (music) of or related to accompaniment
That is intended as a backup.
reinforcements
A copy of a file or record, stored separately from the original, that can be used to recover the original if it is destroyed or damaged.
A reserve or substitute.
An accumulation of material caused by a (partial) obstruction or (complete) blockage of the flow or movement of the material, or an accumulation of material that causes an overflow due to the flow being greater than the maximum possible flow.
Additional; added to supply what is wanted.
Something additional; an extra.