Contract Manufacturer (or co-man, co-manufacturer): Facilities that are contracted by the SQF certified site to produce, process, pack and /or store part of or all of one or more products included in the site’s SQF scope of certification. In some cases, a product may be manufactured interchangeably at the certified site and by the contract manufacturer. In other cases, a contract manufacturer may only be used intermittently to fulfill or supplement the certified site’s production. Contract manufacturers must follow the requirements outlined in the SQF Food Safety Code.
A straight read of the definition for contract manufacturer from the code leaves me thinking the decafeinator in your example is a contract manufacturer. They are a processing part of a product you offer under your scope. They're not a supplier because they're taking your product and returning it. They should be vetted as a co-manufacturer, and their process should be called out in your HACCP flow and HA.