> ## Documentation Index
> Fetch the complete documentation index at: https://docs.arc.io/llms.txt
> Use this file to discover all available pages before exploring further.

# How-to: Select source blockchains for a Unified Balance spend

> Control how USDC is drawn from your Unified Balance across blockchains when you spend, using automatic routing or explicit amounts

A spend pulls USDC from the balances you have deposited on supported
blockchains. Choose one of these approaches to control which blockchains supply
the USDC:

* **Automatic routing**: Specify a total spend amount and let the App Kit SDK
  choose which blockchains to draw from based on your confirmed balances.
* **Explicit amounts**: Set how much USDC comes from each source blockchain
  yourself.

## Prerequisites

Before you begin, ensure that you've:

* [Installed the App Kit SDK](/app-kit/tutorials/installation)
* [Configured an adapter](/app-kit/tutorials/adapter-setups)

These are required so any example below runs with a valid `kit` and `adapter`.

## The App Kit SDK allocates for you

The App Kit SDK chooses how to fund the spend from your confirmed balances. The
kit prefers the destination blockchain first, then pulls from your other
blockchains from highest balance to lowest. Ethereum mainnet is the exception:
it is always last, including when it is the spend destination.

Pass `amount`, `from` with only an `adapter` (no `allocations`), and `to`.

This example spends 2.00 USDC with automatic routing:

```typescript TypeScript theme={null}
const result = await kit.unifiedBalance.spend({
  amount: "2.00",
  from: { adapter },
  to: {
    adapter,
    chain: "Arc_Testnet",
    recipientAddress: "0xRecipientAddress",
  },
});
```

## Explicit per-chain allocations

List each blockchain and how much USDC to draw in `from.allocations`. Each entry
contains an `amount` and `chain`. When you use `allocations`, their amounts must
add up to the top-level `amount`.

This example spends 2.00 USDC (1.00 USDC from Arc Testnet and 1.00 USDC from
Base Sepolia) delivered to the recipient on Arc Testnet:

```typescript TypeScript theme={null}
const result = await kit.unifiedBalance.spend({
  amount: "2.00",
  from: {
    adapter,
    allocations: [
      { amount: "1.00", chain: "Arc_Testnet" },
      { amount: "1.00", chain: "Base_Sepolia" },
    ],
  },
  to: {
    adapter,
    chain: "Arc_Testnet",
    recipientAddress: "0xRecipientAddress",
  },
});
```

You can pass multiple adapters in `from` when you need separate sources (for
example EVM and Solana). Each source uses the same shape: `adapter` and optional
`allocations`. The top-level `amount` must still match the sum of all allocation
amounts you provide.

## Validation rules

* **Amount**: The top-level `amount` is always required. It is the total USDC
  for the spend.
* **Sums**: If you pass `allocations`, the kit checks that they sum to `amount`.
  If they do not match, you get a clear error with both values.
* **Consistency**: Either every `from` entry specifies `allocations`, or none
  do. Mixing sources with allocations and sources without is not supported. If
  you need a computed split first, use `estimateSpend`, then pass the returned
  `allocations` into `spend`.
* **Retry**: Retrying only the mint step after a failure uses a separate
  `retrySpend` flow and parameters, not a partial `spend` call.