In the ease of the decision variables with names longer than four letters, the arrows on the right point to four-letter abbreviations to fit the size limitations of many solvers. The last line indicates that the from plant subscript and to plant subscript are not allowed to have the same value.
There is one more step before writing down the model. To make the model easier to read, it is useful first to introduce macros to represent the summations in the objective function.
The first four macros use the MPL keyword SUM to execute the summation involved. Following each SUM keyword (inside the parentheses) is, first, the index or indexes over which the summation runs. Next (after the colon) is the types of decision variables).
Now this model with 3,100 functional constraints and 21,000 decision variables can be written down in the following compact from.