Then nRF SW backend provides functionality to use the cryptographic implementations from the nRF5 SDK in nrf_crypto. The functionality provided in this backend involves implementations for special use cases, like the code-size constricted version of SHA-256 used in Bootloader.
See Configuring nrf_crypto frontend and backends for details on enabling this backend.
See Cryptography library - nrf_crypto for nrf_crypto APIs that can use this backend.