Often the documentation that the IRS is looking for in your audit will fall into two categories: income side documentation and expense. A review of the IDR that the auditor provided will give you an idea what the focus of the audit is going to be on the income side. The IRS is going to ask you for bank statements and for supporting accounting if applicable that would tend to corroborate the amount of income that you claimed on your return. You can expect to produce a profit and loss if you're a business, you can expect to produce a general ledger and the IRS will also want to see any credit card or any other financial account statements that would tend to corroborate your income. On the expense side they're going to be looking for proof of deductions. So what that means is they're usually looking for receipts, invoices and other documentation that would tend to corroborate your expenses although you can be dinged for missing invoices.
What I like to tell my clients is if we can gather enough data on the expense side and prove that you have generally complete and accurate record-keeping, then the IRS may tend to overlook some invoices in the whole body of work so it's very important to be complete and thorough in your documentation submission to the IRS and to pre audit yourself beforehand so you know if there's any issues with the documentation you're going to be submitting.