The PSA Functional APIs are the developer-facing part of PSA offering security services for building secure end-to-end deployments.
Developers writing the firmware application can rely on these APIs on any PSA-compliant platform regardless of the underlying implementations.
Those interested in portability focuses on pure software solutions, whereas those interested in performance or resistance against physical attacks will favor a hardware-based implementation, potentially hosted inside a secure enclave. Regardless of this, developers can use the same functions, making their code portable and understandable.
The most fundamental APIs are Cryptography, Secure Storage (for secure storage of internal objects, and protected storage for untrusted media) and Attestation.