I have two variables $a$ and $b$, in which $a$ is the amount of goods and $b$ is the amount of boxes of the given sizes. So $b$ (box size + number) is dependent on a (goods quantity). If $a$ is allocated to a given path, then $b$ will increase based on $a$, based on a rule to increase the number of boxes and select the best size.
The objective is to minimize the total cost of using boxes for all paths (i.e. tells the proportion of $a$ that is allocated to
path_n, where the combined cost (spending on boxes) is minimal.)
Problem: How to express the relationship between $a$ and $b$ in the model? Should I model their dependency in the objective function or in the constraint? If constraint, how to describe such a relationship?