include [domain.com/]name
call [domain.com]@name[; $first_var=...; $more_vars=... ; $last_var=...]
external functions can also be called from html (file must be included in a Xtscript first)
<xt:call function="[domain.com]@name" first_var="..." more_vars="..." last_var="..."/>
or like this
<xt:call function="[domain.com]@name" args="$first_var=...;$more_vars=...;$last_var=..." />