The Order IDs in the ISN system are always incrementing. The ISN stores every action and every change to an order so therefore we keep all of that information around.
Thus, for example, if you consume order number 50 (OID 50) with a "test" order - you cannot get that back OID back. The OID of 50 is forever used by that test order. There's no way to permanently get that OID back. Think of them as an odometer on a car; you cannot roll it backwards.
This allows us to always store all of the information your team enters into ISN.
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