Zones let you charge different prices for different parts of the map - e.g. front section vs back. You group seats into zones on the chart, then price each zone in the event's Pricing section.
In the editor, switch to zone mode and click seats or areas to add them to a zone.
In the event's Pricing section, set a price for each ticket type in each zone.
Leave a zone's price blank to use the ticket's base price.
Bix warns you if your mapped seats and ticket capacity don't match, so every available seat is sellable before you publish.