Just a few non-official answers:
1. An extension is installed into Dreamweaver and needs an activation code.
2. A solution is a collection of code that preforms a specific task. Since it is not an extension, it is not controlled by an activation code.
3. Bug fixes are always free. Upgrades that include new features are not included. (i.e. 2.0.1 to 2.0.2 is free, 2.0.1 to 3.0 is not)
4. The subscription gives you the new versions with new features and new extensions as well as the standard bug fixes.