The objective function always includes all of the decision variables, but that is not necessarily true of the constraints

• The objective function always includes all of the decision variables, but that is not necessarily true of the constraints.
• Explain the difference between the objective function and the constraints. Then, explain why a constraint need not refer to all the variables.

Objective function is a linear mathematical relationship that describes the objective of the firm in terms of the decision variables by maximizing or minimizing cost, profit, net present value, risk, customer satisfaction, and number of employees. This is a reflection of the objectives of the operation.

Model constraint is a linear relationship of the decision variables such as daily working hours, raw material availability, and available capacity, that represent the restriction placed on the firm by the operating environment. The restriction can be in the form of limited resources or restrictive guidelines. 

A constraint need not to refer to all the variables because a hypothetical solutions is only to show the solution does not require than restricted. Therefore if not, then there is a feasible solution, and if more is required than restricted, it is a infeasible problem.

Timothy J. Rupert, Kenneth E. Anderson. 2019. Pearson's Federal Taxation 2020 Corporations, Partnerships, Estates & Trusts. Strayervitalsource.com