Pre-planned enhancement to the standard SAP system. SAP anticipates potential customer requests not already implemented in the standard system by predefining them as ’empty modification modules’. Customers fill these modules with their own logic according to their own requirements. SAP creates customer exits using the transaction SMOD. From these customer exits, customers then use the transaction CMOD to choose the enhancements they want to process, assign them to enhancement projects, edit the components, and activate the relevant projects. Using customer exits guarantees upwards compatibility. This is because calls to exits from the standard software and the validity of the call interface are retained, even in future release upgrades.

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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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对标准 SAP 预计划的增强。SAP 预期了那些没有在标准系统中实现的的客户潜在请求，把它们做为 “空的修改模块” 进行了预定义。客户用根据他们自身需求的业务逻辑来填写这些模块。SAP 使用事务码 SMOD 创建客户出口。从这些客户出口，客户就可以使用事务码 CMOD 来选择想要处理的增强，把他们分配给增强项目，编辑这些组件，最后激活相关的项目。使用客户出口保证了向前兼容的能力。因为即使进行了进一步的版本升级，从标准软件对出口的调用和调用接口的有效性还是被保留的。