The diagram roughly illustrates the Synfire family. While Synfire Pro includes every feature ever implemented, the entry level editions Sparks and Express offer a subset of its features optimized for different target audiences at a more affordable price. You can upgrade to a higher edition at any time and can do so incrementally.
Visit the online user manual where you will find a detailed list comparing the features of every edition, along with a summary of what additional features and capabilities you get when you upgrade to the next higher edition.
*) Snippets in Synfire Express can contain only a single phrase at a time, which is very intuitive to get started with because it's similar to what you already know from live loops in various DAWs. Synfire Pro then allows you to have many phrases per snippet, so you can conduct, improvise and arrange different parts live..