Please note: In missions (campaign mode), special industries with different production chains occur.
The numbers in brackets show the (maximum) output of the respective plant's levels. They refer to its respective product, not its base materials! An oil refinery, for example, can process crude oil from two oil wells working to capacity.
The number of arrows refers to the production ratio -- for example, one unit of food is processed out of two units of grain, so you will need the double rate (see line statistics) for grain delivery as for food distribution for an efficient operation. As one farm can only produce 200 units of grain per year, you will need lines delivering grain from four farms at a rate of 200 each in order to get a food processing plant to working to capacity.
If you notice a mistake, please report it to me.