Every foreign operation faces one question before any arithmetic happens. In what currency does this business actually live?
How is the functional currency determined?
It has a name, the functional currency. It's a question of fact, and nobody gets to choose it. IAS 21 and Ind AS 21 define it as the currency of the primary economic environment in which the entity operates, then give indicators in two tiers. The primary ones are the currency that mainly influences the sales prices of goods and services, the currency of the country whose competitive forces and regulations mainly determine those sales prices, and the currency that mainly influences labour, material and other costs. If those three agree, you're done.
Often they don't. The standard then offers secondary indicators: the currency in which financing is raised, and the currency in which operating receipts are usually retained. Where a foreign operation is involved there are four more considerations. These are the ones that decide contested cases. The first is autonomy, meaning whether the operation runs its own activities or works as an extension of the parent. The second is the share of its transactions that are with the parent, high or low. The third is whether its cash flows directly affect the parent's and are readily available for remittance. The fourth is whether it can service its own debt without funds from the parent.
ASC 830 gets to the same place through six economic factors: cash flow, sales price, sales market, expense, financing, and intercompany transactions and arrangements. The wording differs and the outcome rarely does. A self-contained business that sells locally, buys locally, borrows locally and remits a dividend once a year has a local functional currency. A sales office that takes orders in the parent's name, holds no inventory it didn't receive from the parent, and remits everything weekly doesn't.
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Why does the functional currency matter?
Here's why the notes spell this judgement out.
If the functional currency is the local one and the group presents in something else, the statements are translated using the current-rate method, and the difference goes to other comprehensive income where it accumulates in a reserve. Reported profit is the local profit at an average rate, undisturbed. If the functional currency is the parent's, the books were kept in the wrong currency and have to be remeasured into the right one using the temporal method, and the difference goes straight to profit or loss. Reported earnings then carry a line that moves with the exchange rate every period.
Management picks the facts it emphasises. A group that wants smooth earnings prefers a local functional currency, because the currency noise then ends up in a reserve nobody reads. A group whose foreign operation really is an extension of the parent has no choice. The determination is disclosed and auditable, and it's almost never revisited once made. That's worth knowing in itself. A subsidiary set up as a sales office in 2011 and grown into a manufacturing business by 2026 may still carry the 2011 determination.
What changes in a hyperinflationary economy?
There is one place where the judgement is taken away. If the foreign operation's economy is hyperinflationary, both rulebooks intervene, and they intervene in opposite directions. That fight gets its own section.