I can't tell much about it, but I suspect that the variable is made available in the tag and its caller page. The header tag uses its own page context, that would be why you can't access the variable. I don't remember if there's a way to synchronize variables for inner tag files... If you can't find out how to do it, you could use an attribute for the header tag. The spec also recommends that for IN parameters, you should use attributes.