使用时产生。
The Generate From Usage feature enables you to use classes and members before you define them.
功能可以让您在定义类别和成员前，先使用这些项目。
Sometimes the services are free and providers generate revenue from things like Web ads, and other times application providers generate revenue directly from the usage of the service.
有时这些服务是免费的，提供商从网络广告之类的项目中生成收入，有时应用程序提供商直接从使用服务中创造收入。
Sometimes the services are free and providers generate revenue from things like Web ads, and other times application providers generate revenue directly from the usage of the service.
有时这些服务是免费的，提供商从网络广告之类的项目中生成收入，有时应用程序提供商直接从使用服务中创造收入。
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