BIP44_
BIP-44 purpose field.
Per BIP-44: "We define the following 5 levels in BIP32 path: m / purpose' / coin_type' / account' / change / address_index
Purpose is a constant set to 44' following the BIP43 recommendation."
Note: SEP-5 uses a simplified 3-level path (m/44'/148'/n') rather than the full BIP-44 5-level path, as Stellar does not have the concept of change addresses.