Does the 'settings form' property for plugins do anything at all?

I'm mystified.

I wanted to write a relationship plugin to provide a particular context on a panel page, and I wanted the relationship criteria to be configurable so that I could re-use the plugin in other scenarios, so I needed to include a form.

The 'settings form' property is used throughout the ctools plugins code and documentation. The advanced_help documentation describes it; the ctools_plugin_example module implements it; and loads of ctools plugins implement it too.

Given all this, I'm struggling to imagine that it doesn't do anything... but -- in Panels, at least -- none of the 'settings form' forms seem have any effect whatsoever. I subsequently tested every example plugin that I could attach to a panel, and none of their settings forms had any effect on the configuration form for the plugin.

I was going in circles trying to get 'settings form' to work for my relationship plugin, and when I finally traced the code to _ctools_plugin_configure_create_form_info() I learned that the only properties that are actually looked for are 'edit form', 'add form', and 'default form'; none of which are mentioned at all in the documentation for relationship plugins; nor in the docs for any other plugin other than content type plugins.

I implemented 'edit form' and a submit handler in my relationship plugin, and everything is working perfectly.

What am I missing here?