The attributes' value text-align-last = 'justify' forces the fo:block to extend to the available width of the current fo:region-body area. The fo:leader inserts the necessary amount of content of the specified type defined in in leader-pattern to fill up the gap between its neighbouring components. This principle can be extended to multiple objects:
A fo:leader may also be used to draw horizontal lines to separate objects. In this case there are no neighbouring components within the current line in which the fo:leader appears. This is frequently used to draw a border between xsl-region-body and xsl-region-before and/or xsl-region-after:
Note the empty leader < fo:leader /> between the FO and the page 5 text node inserting horizontal whitespace to get the page number centered to the header's right edge. This is in accordance with the leader-pattern attributes default value space.