For a content generated at the beginning of an element, with display: marker, this property allows to align the generated content to the left.

For a content generated at the end of an element, with display: marker, this property allows to align the generated content to the right.

Example (excerpts from DocBook's structure.css):

set:before,
book:before,
part:before,
reference:before,
preface:before,
chapter:before,
article:before,
appendix:before,
section:before,
sect1:before,
sect2:before,
sect3:before,
sect4:before,
sect5:before {
    display: marker;
    marker-offset: fill;
    content: element-name();
    font-size: small;
    color: gray;
}