The user information configuration registers (UICR) are non-volatile memory (NVM) registers that configure user specific settings and values for emulated one-time programmable (OTP).
All UICR registers have a write-once protection that allow them to be read multiple times, but written only once when UICR has been erased by the Erase All operation.
For information on writing registers, see RRAMC — Resistive random access memory controller and Memory.
Notice that all access port protection registers are duplicated into PROTECT0/PROTECT1. For optimal security, set both registers set to "random" values different from the Unprotected value. For ERASEPROTECT, set both PROTECT0/PROTECT1 registers to the Protected value.