This header is of particular benefit in a multi-client environment, as it can be used to delete or update an entry only if it hasn't already been changed since it was last read.
该头部在多客户端环境中尤其有用，因为它可用于删除或更新自上次读取以来未发生更改的条目。
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Update: Some of the Facebook integration in Skype for Windows appeared in October of last year, although Skype today is reporting it as new functionality.