why deploy_nft_item() function's first message is 0x18?

[whyCanNotKK]

() deploy_nft_item(int item_index, cell nft_item_code, int amount, cell nft_content) impure {
cell state_init = calculate_nft_item_state_init(item_index, nft_item_code);
slice nft_address = calculate_nft_item_address(workchain(), state_init);
var msg = begin_cell()
.store_uint(0x18, 6)
.store_slice(nft_address)
.store_coins(amount)
.store_uint(4 + 2 + 1, 1 + 4 + 4 + 64 + 32 + 1 + 1 + 1)
.store_ref(state_init)
.store_ref(nft_content);
send_raw_message(msg.end_cell(), 1); ;; pay transfer fees separately, revert on errors
}

is where rule this number?