1. 64 = carry unused gas, basically forwards gas left from the message to the next transaction.
2. it's easier to understand this via binary representation. anyway, the only thing you must know: use 0x10 for non-bouncable messages, 0x18 for bouncanles ones.

check here for more example on how to craft messages and available modes
https://github.com/TonoxDeFi/open-contracts/blob/main/contracts/messages/messages.func