I think you're mixing two different features here.
The ExtensionManager allows one to add/remove additional functionality to the vCenter with the Register/Unregister methods.
Some examples: Converter, Update Manager, Performance Overview reports...
Some of these Extensions provide plugins for the vSPhere Client.
Most of the time these plugins allow one to interact with the Extension.
But there can also be free-standing plugins.
For example the following XML file will create an entry pointing to a local PowerCLI Reference.
<scriptConfiguration version="1.0.0">
<key>PowerCLI Refrence</key>
<description>PowerCLI Reference</description>
<view parent="Inventory.Global">
<title locale="en">PowerCLI Refrence</title>
<url>file://D:\User\vSphere PowerCLI Cmdlets Reference\index.html</url>
</view>
</scriptConfiguration>
These plugins are computerbased, i.e. if you "enable" a plugin on PC1 it will not automatically be enabled on PC2.
The enabled/disabled state of a plugin is kept in the registry in HKCU\Software\VMware\VMware Infrastructure Client\Preferences\UI.
Each plugin has a REG-SZ key with a of value Enabled or Disabled.
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