It might help to start with an over-simplified version of how the concepts fit together.
- A player is a person who plays an instrument.
- A flow is some music notation that can be played by a group of players, all playing together.
- A layout is a piece of sheet music that contains one or more flows, joined “end to end” (i.e. the music in each flows is played sequentially, not simultaneously), and formatted onto the pages of a document together with titles, page numbers, etc, etc.
- A project in Dorico can contain several layouts (e.g. a full score and a set of parts), and each flow can be included in more than one layout.
There are some extra features you can add to those basic ideas:
- A player might be able to play several instruments, one at a time (e.g. flute/piccolo)
- In a layout, you can select which players from the flow(s) should be included - for example, a full score layout will probably include all the players in each flow, but each part layout will only include one player.
That might help you get started understanding how the pieces fit together! For example if you delete a flow, that means you delete all the music in it, because the only place “music notation” can exist in Dorico is inside a flow. But you can have a flow that isn’t used in any layouts, or a player who isn’t used in any flows - presumably they will be used eventually, or you might decide you don’t need them at all and just delete them from the project.
Part of the confusion might be because when you start a new project in Dorico, you get all these concepts created and linked together automatically. For example, you automatically get a layout for a full score and a layout for each part, each of which contain all the flows in the project, and with the correct player selected for each part layout. That means you can “get started” quickly without really understanding what’s going on, but if you want to do something more complicated than use the default structure for a project, you have to learn how the pieces fit together.