Period rates
As the name implies they are typically defined for an accounting period. They can also be defined for an account code range. For example, you can define several GBP to USD currency period rates, relating to various accounting periods and account ranges.
Period rates are defined using Currency Period Rates (CNP).
They are used by Ledger Entry (LEN) and Ledger Import (LIM) to convert currency values as part of the journal input process. They are also always used by Ledger Revaluation (LER) and financial reporting.
For each currency combination (for a range of accounting periods and/or accounts), you can also define an additional five reporting conversion rates. These rates can be accessed by the report writing facilities and are used to convert values for reporting purposes.