The rate on an invoice
A draft invoice in another currency shows an Exchange rate. Deline looks for an Automatic rate for the invoice date; you can press Find automatic rate again or Enter manually an agreed rate, which Deline marks as a Custom rate. The rate values the invoice in your books and in your tax return. It does not change what your customer owes: the invoice balance stays in its own currency.
Exchange rates in Periods
In Periods, the Exchange rates section keeps the rates your books use, each with its source and evidence. Your books use one named rate source for every currency pair, and once it is saved it cannot be changed. Saving a rate posts nothing. When no published rate is available in the four days up to the end of the period, Use a rate with your own evidence lets you add one with a file and a reason. The owner or the accountant records rates; everyone else can look at them.
Currency adjustments at month end
The Currency adjustments section values open invoices and bills in another currency at the closing rate. Preview valuation shows the result before anything is posted; Value multiple documents & accounts posts one period-end journal per currency. If you lock a month before this is done, Deline says "Some foreign-currency items still need their month-end adjustment. Post it in Currency adjustments, then lock the month again."
What Deline does not do
It does not exchange money or move it between accounts, and it does not change the currency of an invoice after you issue it. The Exchange rates and Currency adjustments sections appear only when your books need them.
Explanation written from the Deline app's own screens and texts (build 81cd9f8f3 (Deline eff8230ba)); it has no steps to check.