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

• "store_uint" is presumably a function or method that takes two parameters.
• The first parameter, "0x18", is a hexadecimal representation of a number. In decimal form, "0x18" equals 24.
• The second parameter, "6", could be the size or length of the data that's being stored, often referring to the number of bits. In this case, the integer 24 is being stored in a 6-bit format.

In general, serialization is the process of converting data structures or object states into a format that can be stored, transmitted, and reconstructed later. This is often used in network communication, when data needs to be sent over a network, or when complex data structures need to be saved for later use.

• https://docs.ton.org/develop/smart-contracts/messages#message-layout