This module just provides a new form element, that can be used by other modules.
Only install it, if another module is instructing you to do so or if you are an interested developer.
This module provides a new form element, that can be used by other modules. This form element allows you to reuse existing forms inside your form!
So you can build forms that reuse existing forms while you extend them with further form items. Note that form reusing means not only reusing the visual representation, but also the validation and submit logic.
If you are interested in the 6.x version, read this.
Status: There won't be any active development in the drupal 5 version or earlier any more.
For a drupal 7 version, have a look at http://drupal.org/project/subform