If a heading (a "heading1" paragraph, for example) is something like this:

This is a very very very long long long heading that could run over several lines.

the chapter field will reflect the whole paragraph, but on my page header I want,
instead of that, the following:

A long heading...

That is, a reduced version. In latex, you can assign a brief "alias" to a very
long heading (something like \Chapter[brief heading]{The very long complete
heading}), so if you use a fancy header, the alias and not the actual heading is displayed in the header.

In OOo, the chapter field displays the whole paragraph, correctly aligned with
line breaks when needed, but there is no way to set an "alias", which is an
important limitation to the design of complex documents.