For buyers contracting authority
Reach for 48517000 when the purchase is a software product, but not one the specific siblings already name. If the licence is for video conferencing, use 48515000; for exchange or messaging server software, 48516000; for a desktop communications tool, 48511000. This code is the fallback for general IT software and software-licensing deals that none of those describe.
The ambiguity to settle first is upward, not sideways. The parent, Communication software package (48510000), and the root family, Software package and information systems (48000000), both look tempting for a broad licensing tender. Pick 48517000 when the contract is a concrete software product or licence package rather than an entire information system or a communications-specific tool.
One more boundary: this code is for the software itself, the licences and subscriptions. Where a contract is mainly about supplying and provisioning software as a service rather than the product, Software supply services (72268000) can be the better fit. Where a single award bundles licences with support or maintenance, set the primary CPV code by whichever element is the dominant purpose of the contract.