Phrases
A bunch of parameters bundled together
Phrase is a structure that bundles one or more Parameters so they can be copied, moved and edited as a coherent unit. Being the primary carrier of musical expression, it is the foundation of Music Prototyping. Its main purpose is to be reused in different contexts to produce different output. A well made phrase can be placed in any Arrangement under any Harmony and "just work".
Although the Figure parameter looks somewhat like a piano roll or fancy MIDI region, its colored symbols are relative (parametric) and thus require least a Harmony parameter in order to be mapped to pitch.
However, since almost every parameter influences MIDI output, there is no way to tell in advance which MIDI notes a phrase will ultimately play before all of an arrangement's Containers and parameters have been considered. That's why Synfire renders all of an arrangement from scratch every time you have changed something and press play.