我们用SAPGUI登录某个系统时，除了用户名和密码外，还要指定一个必填字段client：
这个client是什么东东？
看文档:
SAP Client is the highest hierarchical organizational unit within an SAP system that contains master records and tables. The transactions processed at SAP client level is valid and applicable for all company code data and enterprise structures.Client is a three digits numerical key and from a business point of view, client represents as a corporate group. User is required to enter client key while logging on to the SAP system.
SAP client是SAP系统里包含了主数据和业务数据的层级结构最高的组织单元标识符，业务上来说代表一个企业，技术类型是一个三位的正整数。
Important Client Characteristics
Every SAP user’s access and authorizations must assigned to the SAP client on which client users are working.
It is used to differentiate between development, quality and product system within the same or different server
The data entered at client level is valid for all organizational units.
Each client can have one or more company codes, plants and locations but there will be not have connection between clients.
事务码SCC4查看Netweaver系统上所有已创建的client列表：
