For buyers contracting authority
Pick 48213000 when the contract upgrades or extends an operating system that is already running, rather than supplying a new one. Feature packs, subsystem add-ons, and version-upgrade packages for an installed OS belong here.
The boundary that trips contracting authorities is the sibling 48214000, Network operating system software package. If the deliverable is the operating system platform itself, that is 48214000; 48213000 is reserved for the enhancement that sits on top of an OS the authority already owns. One step up, the parent 48210000 is the right pick when the scope is networking software more broadly and no single sub-category dominates.
Watch the licensing trap too. Where an upgrade is procured mainly as a block of licences and subscription support, the more specific licence-management code (48218000) can be the better tag. Set the primary CPV code by the dominant purpose: if the point is the OS enhancement itself, keep it here; if the point is managing licence entitlements, move it.