For buyers contracting authority
Pick 72253100 when the deliverable is a user-facing point of contact: a first-line or service desk that logs incidents, answers questions, and routes what it cannot resolve. The output you are paying for is responsiveness against service levels, not a piece of software and not a fixed system.
The boundary that trips contracting authorities is the sibling, Systems support services (72253200). If the contract is mainly about keeping servers, networks, and applications running behind the scenes, that is systems support; reserve 72253100 for the desk that the user actually calls or emails. Where a deal bundles both, set the primary CPV code by whichever function dominates the statement of work, and consider the parent (72253000) when neither clearly leads.
One more distinction worth getting right: a helpdesk that answers questions about a specific software product, as part of that product's support, often belongs under Software support services (72261000) instead. Use 72253100 for the general, cross-application desk.