It's just a guide that gives you an idea what the capacity of the rod is.  Some use a weight range, some use line weight, some use both.  There is no "standard" among manufacturers in how they rate their rods.

There is a "proposed" standard that is gaining popularity among the "better" rod builders called the common cents system, developed by a man named William Hanneman.  He actually devised the system using pennies (because they weigh a standard, known, amount and are easy to get).  If one measures a rod or blank using a "known" weight standard they can arrive at an accurate, relative,  comparison against another rod or blank.