Level 3 begins to define the typical business activities that may be conducted within the level 2 entities, such as issuing customer orders, authorizing credit, and tracking order handling.
第3级开始定义在第2级的实体中可能进行的典型业务活动，如发出客户订单、信用卡授权以及跟踪订单处理等。
For documents such as job resumes, product descriptions, and customer orders, XML documents might be a suitable format.
对于工作履历、产品说明和客户订单等文档，XML可能是最合适的格式。
A relation is defined between two tables by specifying key columns that link the tables, most commonly regular relations such as customer-to-orders.
两个表之间的关系通过指定链接表的键列来定义，最常见的常规关系比如“客户-订单”。
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