If the first parameter is greater than 0, will return the second parameter. Otherwise, check() will return the third parameter. See this thread for more details and examples.
check(dbmarket - 1000g, 95% dbmarket, 50% dbmarket)
This is a special function for dealing with item conversions. The convert() function has one required parameter which is the price source to be used in calculating the value of the items which convert to the target item. This parameter cannot be a custom price string and must be simply a price source. Conversions include milling, prospecting, transforming (e.g. greater essences to lesser essences and vice versa) and also vendor trades for inks. The function will return the lowest cost of all possible conversions of the item.
Any price source can be evaluated as a function where you pass an item link or item string in order to evaluate the price source for that specific item. By default, the price source will be evaluated for the item which the entire custom price is being evaluated for. This also works for convert() as the second parameter.
dbminbuyout([Ghost Iron Ore]) matprice([Ink of Dreams]) dbmarket + convert(dbminbuyout, item:79251)