Begin with what you promise customers and how the service could fail to meet those promises. Security is the foundation; discuss additional categories with the auditor based on the system, commitments, and report users' needs. For example, contractual uptime commitments deserve a specific conversation about availability. Handling personal information deserves a careful privacy discussion, not an automatic assumption that every report must include every category. Keep a short rationale for the selection and confirm it with important customers before signing. Adding categories simply to make a report look more complete can create work without answering a real reporting need.
Related: SOC 2 trust services categories.
Further reading: AICPA SOC 2 reporting guide overview.