Hi @bryan_hong
There’s no secure flash which could be used by user. When HSE firmware is installed, certain flash areas are allocated exclusively for HSE resources. These areas are sometimes called as secure flash but users can’t access it, it’s hidden for user.
Users can import keys to HSE. These are stored to secure flash by HSE. This mechanism is dedicated to keys only, it is not recommended to use it for own data. This was already discussed here:
https://community.nxp.com/t5/S32K/Store-User-Specific-value-in-the-secure-NVM/m-p/1856763/highlight/...
As mentioned, usual approach is to encrypt data by a key stored in HSE and also use SMR services for verification of that area.
Regards,
Lukas