You may think it doesn’t matter what companies put on food packaging—come on, a tuna fish with glasses?—but even the smallest details are regulated. For example, the FDA has strict rules on the different turns of phrase manufacturers can print on food labeling, especially when it pertains to ingredients and additives.
"High," "Rich In," and "Excellent Source Of" all mean the same thing: that the product contains at least 20 percent of the daily value per RACC (“reference amounts customarily consumed"—essentially serving size) of whatever it is they’re boasting about.
If a foodstuff is a "Good Source Of," "Contains," or "Provides" something, it must have 10 percent to 19 percent of the daily value (per RACC) of that something.
Meanwhile, when labeling features the words "More," "Fortified," "Enriched," "Added," "Extra," or "Plus," that means that each serving has at least 10 percent of your daily value of that specific thing. However, as the FDA explains, those words are only legally allowed to be used for “vitamins, minerals, protein, dietary fiber, and potassium.”
Some things aren't so easily or pragmatically categorized. Take the word "natural"; the FDA recently asked people to submit their own definitions for the word because it can refer to so many different things. The agency is now in the process of reviewing these submissions as they attempt to nail down a concrete definition. Until then, watch what you eat. Naturally.