Constant scrypto::constants::TRANSACTION_HASHABLE_PAYLOAD_PREFIX

pub const TRANSACTION_HASHABLE_PAYLOAD_PREFIX: u8 = 0x54; // 84u8

The Transaction hash which is signed is created as: hash(TRANSACTION_HASHABLE_PAYLOAD_PREFIX || version prefix according to type of transaction payload || ..)

0x54 = T in ASCII for Transaction - (84 in decimal)