CTools context tutorial
Why should you use panels node-preview-example module? You shouldn't, it's just an example ;)
This post covers how to build a CTools content-type plugin that has context. CTools "Context" should not be mistaken with the context module. Context is CTools way of saying, my plugin (read, block) shouldn't be dumb. If it requires a node or a user to extract data from, then it should let others know about it. "Others" in our case is Page manager module. It will load the context for the plugins and hand it over. In fact, if the context is not present it will not even bother loading the plugin.
I have created an example module called node-preview-example, that you can simply download and enable, create an Article page, and see how Panels is now showing the body field on the left and on the right a (silly) "Summary" of the node -- this is our plugin.