Before revision:  We look at three classes of problems with an external resource: a class of problems where the renting period is budgeted and the scheduling cost needs to be minimized, a class of problems where the scheduling cost is budgeted and the renting period needs to be minimized, and a class of two-objective problems where both, the renting period and the scheduling cost, are to be minimized  .
After revision:  We look at four classes of problems with an external resource: a class of problems where the renting period is budgeted and the scheduling cost needs to be minimized, a class of problems where the scheduling cost is budgeted and the renting period needs to be minimized, and a class of two-objective problems where both, the renting period and the scheduling cost, are to be minimized .