Constraints vs. Objectives: How to Stop Confusing “What Is Allowed” With “What Is Best”

In optimization problems, especially in advertising and bidding systems, people often mix up objectives and constraints.

This confusion is natural. Both are important. Both influence the final decision. Both can involve the same business metric, such as cost, revenue, profit, CPA, ROAS, budget, or conversion volume.

But they play very different roles.

A simple way to remember the difference is:

Constraints define what choices are acceptable. Objectives define what makes one acceptable choice better than another.

In other words, constraints are the rules of the game. The objective is how you win the game.

Let’s make this concrete using the ads bidding domain.

The Ads Bidding Context

Imagine you are running an ad campaign. For each ad opportunity, your system needs to decide how much to bid.

A user visits a website or opens an app. An ad auction happens. Your system estimates:

• How likely the user is to click.
• How likely the user is to convert.
• How much revenue the conversion might generate.