In historical typesetting parlance, a leader is any content that fills an arbitrary area. The most common occurrence of leaders is the row of dots separating section titles from page numbers in a table of contents.
In XSL-FO, a leader fills either a fixed-size area or an
arbitrary area. The area may be blank, or it may be
filled with any of a set of repeating glyphs. The
leader() function returns a
leader element, and has this calling sequence.
length can be any dimension, either as
a fixed string or as an instance of
the default is to produce a leader that fills all the
space left over in its parent element. There are quite
a number of values for the
argument; refer to Pawson's book, pp. 118–119,