Per IRS Instructions for Form 8888 Allocation of Refund (Including Savings Bond Purchases), on page 2:

Purpose of Form

Use Form 8888 if:

• You want us to directly deposit your refund (or part of it) to either two or three accounts at a bank or other financial institution (such as a mutual fund, brokerage firm, or credit union) in the United States, or
• You want to use at least part of your refund to buy up to $5,000 in paper or electronic series I savings bonds.

An account can be a checking, savings, or other account such as:

• An individual retirement arrangement (IRA),
• A health savings account (HSA),
• An Archer MSA,
• A Coverdell education savings account (ESA), or
• A TreasuryDirect^® online account.

You can’t have your refund deposited into more than one account or buy paper series I savings bonds if you file Form 8379, Injured Spouse Allocation.

Deposit of refund to only one account. If you want your refund deposited to only one account, don’t complete this form. Instead, request direct deposit on your tax return.

Account must be in your name. Don’t request a deposit of your refund to an account that isn’t in your name, such as your tax return preparer’s account. Although you may owe your tax return preparer a fee for preparing your return, don’t have any part of your refund deposited into the preparer’s account to pay the fee.

The number of refunds that can be directly deposited to a single account or prepaid debit card is limited to three a year. After this limit is reached, paper checks will be sent instead. Learn more at https://www.irs.gov/refunds/direct-deposit-limits

On page 3:

Direct Deposit Request Rejected

If your financial institution rejects one or two but not all of your direct deposit requests, you may get part of your refund as a paper check and part as a direct deposit.

Example. You complete lines 1 and 2 correctly but forget to enter an account number on line 3d. You will get a paper check for any amount shown on line 3a. The parts of your refund shown on lines 1a and 2a will be directly deposited to the accounts you indicated.

CAUTION! The IRS isn’t responsible for a lost refund if you enter the wrong account information. Check with your financial institution to get the correct routing and account numbers and to make sure your direct deposit will be accepted. See Reasons Your Direct Deposit Request Will Be Rejected under Refund in the Instructions for Form 1040 for more information.

Note that any link in the information above is updated each year automatically and will take you to the most recent version of the webpage or document at the time it is accessed.