Application Compatiblity Toolkit has long suffered from insufficient documentation - Microsoft has recognized this and has updated the docs...
This documentation update focused on documenting what specific shims do and gives usage scenarios and examples. If you've been struggling with this toolkit - this should help you out immensely. Chris Jackson and his team have not gotten through all the shims, but the ones they've addresssed look very good.
It's a little tricky to find this update because it is posted as an additional document download on the existing Application Compatibility Toolkit 5 download (dated 11/12/2007) and it has an odd name. You want to download the document titled "ACT50_Doc_Update_Installation_Instructions.zip" from the link below.
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Topic: Engineers Toolbox, Application Management, Vista