If your board is running in Pay Mode, then you may identify each category by a specific price level. The price level multiplier affects the base price for posting an ad. If for instance you have $5.00 base price for posting an ad, then assigning [1] as a price level for category "Jobs" would leave base price on $5.00 level for posting in "Jobs". But [1.5] price level for category "Rentals" would set the base price as $7.50 for posting in "Rentals". Price levels adds flexibility to pricing scheme depending on selected category.

Note: Price Level is just a quick way to differentiate the ad price depending on category. There are more advanced "Pricing Schemes" and "Packages" methods described in Payment System and Board Operations.