Constant INITIATION_OPTIONS_SHIFT 

const INITIATION_OPTIONS_SHIFT: u32 = u64::BITS; // 64u32

Bit position of the 64-bit options word within StartInitiation::additional_data.

The options word (PIX parameters and SURB balancer target) occupies the most significant 64 bits; the least significant 64 bits are currently unused and always sent as 0.