Extra currencies
In TON, extra currencies are arbitrary custom tokens that can be stored alongside TON in an account’s balance.
Extra currency balances can be stored on each blockchain account (ExtraCurrencyCollection) and transferred to other accounts natively (in an internal message from one smart contract to another, you can specify a hashmap of the extra currency amounts in addition to the Toncoin amount).
//extra currency part of account balance
extra_currencies$_ dict:(HashmapE 32 (VarUInteger 32))
= ExtraCurrencyCollection;
currencies$_ grams:Grams other:ExtraCurrencyCollection
= CurrencyCollection;
//account balance kept in account storage
account_storage$_ last_trans_lt:uint64
balance:CurrencyCollection state:AccountState
= AccountStorage;
account_none$0 = Account;
account$1 addr:MsgAddressInt storage_stat:StorageInfo
storage:AccountStorage = Account;
A hashmap of currency IDs and amounts.
However, extra currencies can only be stored and transferred (like Toncoin) and do not have their own arbitrary code or functionality.
Note that if there are a large number of extra currencies created, account state size will increase because they need to store them.
Thus, extra currencies are best used for well-known decentralized currencies (for example, Wrapped Bitcoin or Ether), and creating such an extra currency should be quite expensive.
Jettons are suitable for other tasks.
At the moment, no extra currency has been created on TON Blockchain. TON Blockchain has full support for extra currencies in accounts and messages. See the minting flow for details.