It's just a unit, a way of thinking about what you eat. Unfortunately, food packages don't all reflect the USDA philosophy. The consumer needs to do a little rough math and measuring in the market and the kitchen. The USDA suggests a daily diet of 6 to 11 servings of bread, cereal, rice or pasta. All these foods are based on grain, so call this the grain group.
Cold cereals are different.
Their labeled servings generally fall into two styles. The Pyramid suggests 3 to 5 servings of vegetables every day. This is surprisingly easy to satisfy since a mere cup of vegetables, cooked or raw, equals 1 serving. Raw leafy fare such as spinach or romaine in a salad calls for a full 1 cup. We need 2 to 4 servings of fruit.