In the Logic panel in Blender, you can set properties, which will be translated to tags. A tag is nothing more than an arbitrary property assigned to a node. You can access these from the game using getTag/getNetTag. (Some special properties, like ‘Collide’, have different meanings and are not exported into tags by Chicken.)
I advise looking at Naith. It uses a really cool technique - in Blender, it links objects like trees to the level file in Blender, which you can instance into the level and instantly see the result, while you can still modify the tree separately (and it will automatically update in the level .blend file).
The trees will be exported as empties to the .egg file, and the .egg versions of those trees are put in place from the code. (Although I believe Chicken also supports {{}} structures that reference other .egg files).