The field is unavailable because an encrypted field cannot be used directly as the criterion in a criteria-based sharing rule. This is a platform limitation arising from the security properties of encrypted data, not a consequence of the architect ' s personal Field-Level Security. Granting the administrator more visibility to the field therefore would not make the field eligible for sharing-rule criteria. The design implication is important: encryption can affect downstream functionality such as filtering, searching, deterministic comparisons, reporting, and sharing-rule logic. If UC needs sharing behavior related to the sensitive Social Security Number, the architect should avoid duplicating the plaintext value merely to bypass the restriction. A safer design is to use a non-sensitive classification, segment, status, or derived security indicator that expresses the business condition without exposing the protected identifier. The encryption decision and the sharing design must therefore be considered together rather than independently. Study Guide reference: Implications of Security Model Choice - Shield Platform Encryption limitations, criteria-based sharing, sensitive-data architecture, and security design trade-offs.