In the ITP Model you can see these instructions:
(* Set headers and footers *)
TEXT header
TEXT footer := "Page 1" (* <- Page numbering field from Word *)
TEXTBLOCK
VAR label + "\Header"
ASSIGN_TO header
#@(headers(header; ""; ""))#
#@(footers(""; footer; footer))#
First, two variables are declared for the headers and the footers. The footer variable immediately receives its value; the word Page followed by a Microsoft Word field that will show the page number. The header variable is filled with the contents of a Rich Text Block. The location of the Rich Text Block is determined by the value of the variable that contains the earlier selected label. If label A is selected, the Brand A logo appears on the top right of the page.
The results of the functions headers and footers are produced as output in the result document. The function pagestyle is called first, which creates a new section. The functions headers and footers are called afterwards, so the text specified is attached to the headers and footers of the newly created section.
Both functions have three parameters that allow you to pass the contents of the three different headers and footers within a section:
You have to be aware that the appearance of the content in the headers and footers also depends on the page settings in the result document. Even if you pass three different values as headers, they will only appear if the page properties in the pagestyle template document have the options enabled that show different headers for first, even and odd pages. In the image below you can see the layout tab of the page setup dialog as presented in Microsoft Word 2003. The options "Different odd and even" and "Different first page" for headers and footers are enabled.

More information on headers and footers can be found in the related topics of the model developer's Help.