The documentation for convertCurrency() says this:
“If an org has enabled advanced currency management, dated exchange rates are used when converting currency fields on opportunities, opportunity line items, and opportunity history. With advanced currency management, convertCurrency uses the conversion rate that corresponds to a given field (for example, CloseDate on opportunities).”
A "given field" of CloseDate is mentioned for Opportunity records, but I have not been able to find anything documented about how to find what field is used for an object.
CloseDate is mentioned in the example for Opportunity, but what about Opportunity Line Items? And other non-Opportunity-related objects?
And is CloseDate always the field used related to Opportunity?