DAX statement with Ifs and nested SUMIFS

OK.

Classification = 
var __currentAccountNumber = 'Accounts Receivable'[Account Number]
var __currentDebt = 'Accounts Receivable'[Debt USD]
var __currentInvoiceType = 'Accounts Receivable'[Invoice Type]
var __currentTimeInterval = 'Accounts Receivable'[Time Interval]
var __sumOfDebtForCurrentClient = 
    SUMX(
        FILTER(
            'Accounts Receivable',
            'Accounts Receivable'[Account Number] = __currentAccountNumber
        ),
        'Accounts Receivable'[Debt USD]
    )
var __classification =
    SWITCH( TRUE(),

        __sumOfDebtForCurrentClient < 0, 
            "Credit Balance",
            
        __currentDebt < 0 && __currentInvoiceType in {"AA", "BB", "CC", "DD"},
            "Unapplied Payment",

        // else
        __currentTimeInterval
    )
return
    __classification

This is your formula for the Classification Column in your table.

If this is not performant enough, then here's a variation on this topic. Just change these lines:

var __sumOfDebtForCurrentClient = 
    SUMX(
        FILTER(
            'Accounts Receivable',
            'Accounts Receivable'[Account Number] = __currentAccountNumber
        ),
        'Accounts Receivable'[Debt USD]
    )

to these

var __sumOfDebtForCurrentClient =
    CALCULATE(
        SUM( 'Accounts Receivable'[Debt USD] ),
        ALLEXCEPT( 
            'Accounts Receivable',
            'Accounts Receivable'[Account Number] 
        )
    )

Not sure which one would be faster on a big table.

By the way, here's the Excel formula (which seems to implement a different logic than the one you've described), courtesy of http://excelformulabeautifier.com/:

=IF(
    IF(
        [@group] <> "CASH",
        SUMIF(
            [accountNumber],
            [@accountNumber],
            [debtUSD]
        )
    ) < 0,
    "Credit Balance",
    IF(
        AND(
            [@[debtUSD]] < 0,
            OR(
                [@[invoiceType]] = "AA",
                [@[invoiceType]] = "BB",
                [@[invoiceType]] = "CC",
                [@[invoiceType]] = "DD",
                [@[invoiceType]] = "EE",
                [@[invoiceType]] = "WV"
            )
        ),
        "Unapplied Payment",
        IF(
            OR(
                [@[collectionsAgent]] = "JohnDoe"
            ),
            "Legal",
            [@[timeInterval]]
        )
    )
)

But I've implemented the logic you've described :)

Best

Darek