Phantom supports Versioned Transactions and Legacy Transactions. Versioned Transactions and Address Lookup Tables (LUTs) were introduced by Solana in an effort to improve the developer and end-user experience. The proposed changes are as follows:
  • Introduce a program which manages on-chain address lookup tables
  • Add a new transaction format which can make use of the above on-chain address lookup tables
Why do we need the above? Legacy transactions have a major issue: Maximum allowed size of 1232 bytes, and hence the number of accounts that can fit in an atomic transaction: 35 addresses. Legacy Transaction Avi This is problematic as developers are limited with this threshold and are unable to include >35 signature-free account / program addresses in a single transaction. This is where Address LUTs come into the picture.

​
Address LUTs

The idea behind Address LUTs is to store account addresses in array-like data structures on-chain. Once accounts are stored in this table, the address of the table can be referenced in a transaction message using 1-byte u8 indices. Address Lookup Tables Avi This opens up space as addresses need not be stored inside transaction messages, but only be referenced. This allows 2^8=256 accounts to be included in 1 tx, as accounts are referenced using u8 indices.

​
RPC changes

Transaction responses will require a new version field: maxSupportedTransactionVersion to indicate to clients which transaction structure needs to be followed for deserialisation. The following methods need to be updated to avoid errors:
  • getTransaction
  • getBlock
The following parameter needs to be added to the requests: maxSupportedTransactionVersion: 0 If maxSupportedTransactionVersion is not explicitly added to the request, the transaction version will fallback to legacy. Any block that contains a versioned transaction will return with an error by the client in the case of a legacy transaction. You can set this via JSON formatted requests to the RPC endpoint like below: 
curl http://localhost:8899 -X POST -H "Content-Type: application/json" -d \
'{"jsonrpc": "2.0", "id":1, "method": "getBlock", "params": [430, {
  "encoding":"json",
  "maxSupportedTransactionVersion":0,
  "transactionDetails":"full",
  "rewards":false
}]}'
You can also do the same using the @solana/web3.js library. 
// connect to the `devnet` cluster and get the current `slot`
const connection = new web3.Connection(web3.clusterApiUrl("devnet"));
const slot = await connection.getSlot();

// get the latest block (allowing for v0 transactions)
const block = await connection.getBlock(slot, {
  maxSupportedTransactionVersion: 0,
});

// get a specific transaction (allowing for v0 transactions)
const getTx = await connection.getTransaction(
  "3jpoANiFeVGisWRY5UP648xRXs3iQasCHABPWRWnoEjeA93nc79WrnGgpgazjq4K9m8g2NJoyKoWBV1Kx5VmtwHQ",
  {
    maxSupportedTransactionVersion: 0,
  },
);
For details on how to build and send Versioned Transactions and LUTs, see Send a versioned transaction.

​
Further reading