Setting nSequence to this value for every input in a transaction disables nLockTime/IsFinalTx(). It fails OP_CHECKLOCKTIMEVERIFY/CheckLockTime() for any input that has it set (BIP 65). It has SEQUENCE_LOCKTIME_DISABLE_FLAG set (BIP 68/112).
Declared in <primitives/transaction.h>
inline constexpr static
uint32_t SEQUENCE_FINAL = {0xffffffff};