Rules
A rule is a named set of conditions and actions. Conditions can match payee (contains, equals, or regex), amount range, account, and inflow or outflow. Actions can rename the payee, set a category, add a tag, mark a transfer, or split by a fixed amount or a percentage.
When rules run
Rules run on imported and downloaded items. They never run silently on a transaction you type in the register. Each applied action shows the rule name and sits on the undo stack.
Use Apply to imported if you saved a rule after the file was already in the ledger.
Preview
Preview lists historical rows the draft would match. Nothing is written until you save the rule and apply it to imported items.
Suggestions
If you assign the same category to the same payee twice, the Inbox and the Rules page suggest a rule. You review it before it exists.