The title2 argument can be given to any element that has the title argument.
Its usage is a bit like the description= argument of images, allowing you to add some extra content to the header without affecting its ID.
Unlike description= however, title2 shows up on all full references, including appearances in the table of contents, which make it more searchable.
Its primary use cases are:
  • give acronyms, or other short names names of fuller titles such as mathematical/programming notation
    One primary reason to not use the acronyms as the main section name is to avoid possible ID ambiguities with other acronyms.
  • give the header in different languages
For example, given the OurBigBook input:
= Toplevel

The Toc follows:

== North Atlantic Treaty Organization


the rendered output looks like:
= Toplevel

The ToC follows:

* North Atlantic Treaty Organization (NATO)

== North Atlantic Treaty Organization (NATO)

North Atlantic Treaty Organization

Section 1. "North Atlantic Treaty Organization (NATO)"
Related alternatives to title2 include:
Parenthesis are added automatically around all rendered title2.
The title2 argument has a special meaning when applied to a header with the \H synonym argument, see \H title2 argument of a synonym header.