Money Transfers move money between your own bank and cash accounts — bank to petty cash, one bank to another, one currency to another. A transfer names the from-account, the to-account, the amount sent and — when the two accounts' currencies differ — the amount received and the effective rate, because what leaves one account is not always what arrives in the other. The difference is real money (charges, spread) and the books show it rather than smoothing it away.
The form helps you land those figures honestly. Amount sent carries the from-account's currency on the field, and Amount received carries the to-account's, so there is never a doubt about which currency a number is in. Once both accounts are picked and the amount is typed, the form fills the effective rate (destination units per 1 unit sent) from your exchange rates — directly, reversed, or crossed through your home currency, whichever your rates support — and suggests the amount received at that rate. Both stay editable: if the bank applied its own rate, type it and the received amount follows your rate; if the bank moved a different amount, type that instead — what you type is never overwritten. And if no rate in your table can connect the two currencies, the rate and received fields stay blank rather than inventing a figure: enter the amount that actually arrived, and add the rate under System Setup → Exchange Rates when you have it.
Expense Payments pay for something directly, without a supplier invoice first: a payee, the bank or cash account it is paid from, and lines splitting the total across expense accounts (and departments, when you track them). Rent day and the plumber are expense payments; a supplier's bill is not — that travels the Procurement road, so the creditor's detail exists (Chapter 4).
Both documents post through the same overdraft guard as supplier payments: if paying would take the account below zero, your company's overdraft policy decides — block (the document stays a draft, the reason named) or warn (an explicit warning names the overdraft, and proceeding is a recorded decision, not an oversight).