From http://www.w3schools.com (Copyright Refsnes Data)

XSL-FO layout-master-set Object

Definition and Usage

The <fo:layout-master-set> object holds all masters used in a document.

Note: <fo:root> is the parent of <fo:layout-master-set>.

Note: The master-name of each child of the <fo:layout-master-set> must be unique within the document.

Syntax

<fo:layout-master-set>
  <!--
	Contents:(simple-page-master|page-sequence-master)+
  -->
</fo:layout-master-set>

Properties

NONE

Example 1

XSL-FO documents have a structure like this:

<?xml version="1.0" encoding="ISO-8859-1"?>

<fo:root xmlns:fo="http://www.w3.org/1999/XSL/Format">

<fo:layout-master-set>
  <fo:simple-page-master master-name="A4">
    <!-- Page template goes here -->
  </fo:simple-page-master>
</fo:layout-master-set>

<fo:page-sequence master-reference="A4">
  <!-- Page content goes here -->
</fo:page-sequence>

</fo:root>