I have a function:

<cffunction name="MyFunction" output="false" returnType="boolean">
<cfargument name="thisArg" type="string" default="">

<cfquery name="qryWhatever" datasource="dsnWhatever">
SELECT Field1, Field2
FROM ThisTable
WHERE Field3 = '#thisArg#';
</cfquery>

<cfset srcEmail = 'nameATdomain.com'>
<cfset destEmail = 'nameATdomain.com'>
<cfinclude template="MyInclude.cfm">
<cfreturn true>
</cffunction>

Within MyInclude.cfm:

<cfmail from="#srcEmail#" to="#destEmail#" subject="whatever">
Hello #thisArg#
</cfmail>

The values of srcEmail and destEmail seem to work correctly, but I get an error saying thisArg is undefined when I first use it in the include file. However I can see from my debugging info that the value of thisArg existed within the UDF that called the include. Is this a scope issue? I just noticed I hadn't explicitly (with the var keyword) limited srcEmail and destEmail to the local scope in the UDF. But since the include is within the UDF, should it not have access to any value within the UDF's scope as well?

Found my own answer. When I set srcEmail and destEmail to local scope with:

<cfset var srcEmail = 'nameATdomain.com'>
<cfset var destEmail = 'nameATdomain.com'>

an error was generated whenever they were first encountered in the included file as well. So it is a scope issue. I ended up copying the argument value into another variable, not defined with var, and it worked ok. So I ended up with:

<cfset srcEmail = 'nameATdomain.com'>
<cfset destEmail = 'nameATdomain.com'>
<cfset argVal = thisArg>
<cfinclude template="MyInclude.cfm">

In case anyone was curious. If any of you own/use Mastering ColdFusion MX, by Danesh, Camden, Bainum, and Rish (ISBN 0-7821-4124-2), this appears to directly contradict the text on page 89:

More subtleties exist when multiple templates are involved. When you used cfinclude, the code of the included template became part of the calling template. This mean it is part of the same scope. The code of the calling template can directly refer to variables contained in the code of the template being included and the other way around.