Which two elements of financial management for IT services are mandatory?

Charging is not a mandatory element of Financial Management; it is optional and depends on whether the organization chooses to implement a chargeback or showback model.

Accounting is mandatory, but charging is optional, so this pairing does not represent the two mandatory elements.

ITIL defines Financial Management as comprising three sub-processes: budgeting, accounting, and charging. Of these, budgeting (planning future financial spend) and accounting (tracking actual IT spend and costs) are mandatory for all organizations, whereas charging - recovering costs from customers - is optional and only implemented when appropriate to the business model.

Costing is an activity within accounting rather than a standalone mandatory sub-process, and charging remains optional.