上QQ阅读APP看书，第一时间看更新
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In step 3, you used the $PSItem mechanism to represent an object in the pipeline. You could have used $_ instead. Either works.
To understand the various categories of updates, review the descriptions available and refer to the online documentation:
$WSUSServer.GetUpdateClassifications() | Select-Object -Property
Title,Description
Documentation of the terminology that defines the software updates is available at https://support.microsoft.com/en-us/help/824684/description-of-the-standard-terminology-that-is-used-to-describe-microsoft-software-updates.