Peripherals with split security are defined to handle use-cases when both secure and non-secure code needs to control the same resource.
When peripherals with split security have their security attribute set to non-secure, access to specific registers and bitfields within some registers is dependent on the security attribute of the bus transfer. For example, some registers will not be accessible for a non-secure transfer.
When peripherals with split security have their security attribute set to secure, then only secure transfers can access their registers.
See Peripherals for an overview of split security peripherals. Respective peripheral chapters explain the specific security behavior of each peripheral.