In order to use the same number of bits to encode roughly the same wall-clock duration, and because blocks are naturally limited to occur every 600s on average, the minimum granularity for time-based relative lock-time is fixed at 512 seconds. Converting from CTxIn::nSequence to seconds is performed by multiplying by 512 = 2^9, or equivalently shifting up by 9 bits.
Declared in <primitives/transaction.h>
inline constexpr static
int SEQUENCE_LOCKTIME_GRANULARITY = {9};