Money out mirrors money in: the purchase order (what you asked for), the bill (what you owe), and the bill payment (what you sent).
Entering a bill
Open Bills and create one. Pick the vendor, the date and the due date, and add lines with their expense categories. If the bill arrived as a PDF, attach it: the document stays with the bill forever, so the original is always one click from the entry it explains. A purchase order you already entered can fill the bill in for you.
Paying
Pay bills lets you settle one or many at once: tick the bills, choose the account the money leaves from, and record it. Paying by paper? kBooks prints checks on standard check stock, and the check register under Transactions keeps the numbering straight.
When the payment clears the bank, the feed line matches the payment you recorded instead of creating anything new, so nothing doubles.
Recurring costs
Bills that repeat (rent on your office, software, insurance) can be memorized so each period's copy appears on schedule, ready to confirm rather than retype.
Keeping an eye on what you owe
The Overview shows what is due and what is overdue, and the payable aging under Reports breaks it down by vendor and by how long it has waited. Each vendor's page carries their whole history: bills, payments, documents and notes in one place.