For a company car instalment payment, which classification code should I use?

No classification code is required.

Instalment payments are loan repayments to a bank or financial institution, where the instalment consists of:

a. Principal → Loan repayment (no e-Invoice required).

b. Interest → The bank will issue the e-Invoice (usually via periodic statements or bills).

You may retain your current business practice — for example, create a payment voucher to record the loan repayment and interest charged (as separate items), or record them in supplier bills and knock off with paybills — both of which do not require a classification code.

Recommendation: Keep the bank’s loan agreement and statements as supporting documents for both principal and interest payments.