As humans, we have receptors for two kinds of beta adrenergic compounds such as catecholamines. Cardiac muscle cells have beta 1 receptors that promote increased heart rate. Some drugs that slow heart rate are called beta blockers. Smooth muscle cells, however, have beta 2 receptors which mediate smooth muscle contractions. Blockers of these effects are sometimes used to treat asthma.

The description above illustrates what?

Based on the scenario being described within the question it can be said that this description best illustrates that just because a drug acts on one type of receptor does not mean that it will act on another type. That is because there are a variety of different receptors each of which target different areas and have different functions, therefore a drug that performs a specific function cannot be used on all receptors.