Dorico provides three types of layouts:
A full score layout includes by default all players and all flows in your project. You can remove players and flows that you do not need.
An instrumental part layout is automatically created when you add a player to your project. You can add further players to the instrumental part layout. You can also create empty instrumental part layouts and add players. By default, an instrumental part layout contains all flows. You can exclude flows that you do not need.
A custom score layout initially does not contain any players or flows. This allows you to create your score manually and to add, for example, a layout that contains only one flow instead of all flows.