A system of marks is used to represent convenient units in a particular context. The adavantage of using marks is that it helps you to graph adequately relation between different units. The important thing here is that each unit of the particular quantity (distance, time, weight, years, height, etc) is represented by one mark. Thus, one unit is one mark, two units are two marks, etc.
Furthermore, it is also possible to one mark represent 2 units, 3 units, or in general whichever amount of units. In this case, each mark is an interval between two fixed numbers. For example, if you have that one mark represents 5 units (it is made because there are a lot of situations in which the information requires bigger numbers), then, two marks will represent 10 units, three marks will represent 15 units, etc.