Which term describes if a service is fit for use?

Serviceability refers to the ability of a third-party supplier to meet contractual terms for maintaining service components, not whether the service itself is fit for use.

Utility describes whether a service is fit for purpose - what the service does to support business outcomes - not how reliably it is delivered.

Warranty in ITIL is the assurance that a service will meet agreed requirements and is fit for use. It encompasses assurances about availability, capacity, continuity, and security, answering not what the service does, but how reliably and safely it is delivered.

Availability is only one component within Warranty, referring to the proportion of agreed time a service is accessible, not the broader concept of fitness for use.