Regulation S-X Rule 5-02.4 requires the allowance for doubtful accounts and notes receivable to be stated separately, on the balance sheet or in a note. Since the current expected credit loss model in ASC 326 (known as CECL) took effect, companies measure the allowance based on losses they expect over the whole life of the receivable, using past experience, current conditions, and reasonable forecasts. The older approach recorded a loss only once it had become probable. In XBRL filings the current allowance on trade receivables is tagged AllowanceForDoubtfulAccountsReceivableCurrent, and the period's charge to earnings is tagged ProvisionForDoubtfulAccounts.
Movements in the allowance follow a simple roll-forward: the opening balance, plus the provision charged to expense, minus write-offs, plus recoveries of amounts previously written off. Companies disclose this roll-forward in their footnotes. A write-off by itself does not hit earnings; the income-statement cost was already taken when the provision was recorded.
Because the size of the allowance depends on management's judgment, it is one of the places earnings can be shaped. Cutting the provision raises reported profit in the current period. Analysts compare the allowance with gross receivables over time and with peers. A falling reserve ratio while receivables are aging, or while days sales outstanding is rising, suggests the company may be under-reserving. A sudden jump often marks a customer in distress or a weaker economic outlook.