Logical process in the R/3 System. From the user’s point of view, a transaction is a self-contained unit (for example, generate a list of customers, change the address of a customer, book a flight reservation for a customer, execute a program). From a dialog programming point of view, a transaction is a complex object that consists of a module pool and screens, and is called by specifying a transaction code. After logon, there are 3 distinct levels in the R/3 System: R/3 level, Work area level, and Application level. A transaction is an application at application level. To go to the initial screen of an application, you can either navigate through the menus by choosing the appropriate menu options or enter the appropriate 4-character transaction code in the command field. Using the transaction code saves navigation through the menus and takes the user directly to the initial screen. Example: To call the ABAP Editor from the initial screen of the R/3 System, you either choose Tools -> ABAP Workbench -> ABAP Editor, or enter the transaction code SE38 in the command field.

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分隔线上面是 SAP 标准文档中提供的说明
分隔线下面是我的翻译
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R/3 系统中的逻辑流程。从用户的观点来看，事务是一个自包含的单元（例如，生成一个客户清单、修改客户的地址、为某个客户预定航班预留、执行一个程序）。从会话编程的观点来看，事务是一个包含了模块池和屏幕的复杂对象，被指定的事务码调用。在登录后，R/3 系统有三个清晰的层级：R/3 层级、工作区层级和应用层级。事务是应用层的一个应用。要转到应用的初始屏幕，可以通过菜单选择相应的菜单项或者在命令字段输出事务码来操作。例如：要从 R/3 系统的初始屏幕调用 ABAP 编辑器，可以选择 “工具－ABAP 工作台－ABAP 编辑器”，或者在命令字段输入事务码 SE38。