Flows are separate spans of music that are completely independent in musical content; for example, a single song in an album, a movement in a sonata or symphony, a number in a stage musical, or a short scale or sight-reading exercise of only a few bars in length. A single project can contain any number of flows.
Each flow can contain music for any combination of players, independently of other flows. For example, brass players are often tacet in the second movements of Classical-period symphonies, so you can remove brass players from the flow for the second movement but leave them in the flows for other movements. In a set of cues for a movie, for example, specific players might not be required in some cues, so the corresponding flows can contain only those players who have anything to play.
The correct assignment of players to flows allows Dorico, for example, to generate tacet sheets automatically for individual instrumental parts.