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".

Figure 1. Example Phrase

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.

Important: Phrases may carry only a single parameter, like Velocity or Pause. Keep in mind that if you move a phrase around, it will replace an existing phrase including all its parameters. So, if you want to copy or move a single parameter to another phrase, be sure to drag it off of a Parameter Outlet instead of grabbing the entire phrase.
Note: The term Phrase stands for a technical data structure here, not to be confused with the meaning it has in classical music theory.