MSBuild ProjectReference:private ("Copy Local") - what are the allowed values and behaviour?

For Reference and ProjectReference items, the accepted values for Private are: True or False

This property in msbuild corresponds with the project reference property in VS as Copy Local.

I got the above answer by manually setting the reference properties in VS and viewing the xml. I couldn't find official documentation of the Private item metadata.

Checking the docs at https://msdn.microsoft.com/en-us/library/bb629388.aspx shows the accepted values as Never, Always, and PreserveNewest. These seem to be wrong and only available for the CopyLocal metadata, which is used on Content, None, and other file items.