Tax inclusive vs tax exclusive amounts

Actually OK. I’ve worked out I need to use the templates.
But how does all the tax stuff work? I can follow HTML but I can’t follow the code on the template.
When I hit preview on a test invoice that has witholding tax none of the witholding tax shows up on the invoice at all???

Why is there no information about any of this in the Guides considering how much of the bleeding obvous IS actually explained?