Since the schema represents the abstract knowledge, each part of which is filled by various scenarios. For example, the “waiter” in the restaurant schema can be acted by different people such as a male or a female, young or old, Paul or Jim, so “waiter” is a variable. The human knowledge depends on the schema in the memory. Facing the colorful world and different inpiduals, schema can tell you what it is. You will not be able to know about the world without the schema. But the kinds of the schema is constrained, not all the information can fill in a specific space. The kinds of schema can restrict the information in a certain confines. Take the “waiter” as an example again, he can only be acted by people, not animals, plants, or other objects and not everyone can acted the specific “waiter”, he can’t be either too old or too young. But is there a standard? It is difficult to have an certain standard but a vague boundary. Vagueness is one feature of variable constraint and another feature of which is that variable has a typical value. In other words, the one filled in the space is the most typical information in this schema variable. And of course the change of the variable schema constraint is not random, it must accord with the reliability of the objective reality. Variable constraint has vital significance on cognition.
    Schema has hierarchies in which big schema contains small one, and small schema contains a much smaller. Certainly, if it is too small, it may not have any smaller level, and which can be called the basic schema. We have larger schema such as event schema, scene schema, role schema and concept schema.
2.4 Functions of Schema Theory
Reader’s schemata have a great influence on comprehending of the information and the ideas of a text. Thus, it’s important for teachers and students to understand how the schema works and how it contributes to reading comprehension. According to Anderson, schema theory has some functions in the following.  论图式理论在初中英语阅读教学中的应用(4):http://www.751com.cn/yingyu/lunwen_38326.html