Why Is SHA256 Used for Signature Verification of the Pre-Ordering Request, But SM2 Is Used for Signature Verification of the Payment Callback? Can Only SM2 Be Used for Signature Verification?

For the merchant's pre-ordering request, Huawei Pay provides two signature modes: SHA256WithRSA/PSS and SM2. The merchant can choose either of them.
For payment services, the SM algorithm must be supported to meet regulatory requirements. Huawei Pay uses SM2 to sign callback notifications. Therefore, only SM2 can be used for callback signature verification.