Final answer:
The REX code used to indicate a cancelled requisition from the SOS is 'CX'.
Step-by-step explanation:
The REX code that is used to indicate that a requisition was cancelled from the SOS is 'CX'. When a requisition is cancelled, the CX code is added to the REX (Requisition Exchange) file. This code is used to identify and track the cancelled requisitions within the system.