It’s hard to give a chapter and verse answer on this, since all of the playback functionality is very much in its infancy at the moment.
Allow me to introduce the term “endpoint”, which may well not end up as a user-facing term in the application, but which is nevertheless for us an important concept. An endpoint is a combination of virtual instrument (device), channel, patch (if known), and any required switch information (e.g. a particular key switch or a specific MIDI controller set within a particular range, etc.), that produces a playing technique for an instrumental sound.
If you’re using the supplied HALion Sonic SE 2 player and its own factory content or HALion Symphonic Orchestra (HSO), then the assignment of instruments to endpoints is automatic: Dorico examines the instruments held by the players, and knows which playing techniques are provided by the HSO instruments, so it can instantiate as many instances of HALion as it needs and load the appropriate HSO patches automatically.
If, however, you’re using any other virtual instrument, then this allocation of instruments to endpoints has to be done manually. You might, for example, load up Vienna Ensemble Pro and load it up with 16 channels of patches. Dorico can’t see inside VE Pro, so it doesn’t know what patches you’ve loaded there (beyond their names), and nor does it know anything about the switches those patches may use to access different playing techniques.
If a suitable expression map is available, then you’ll be able to link that somehow to the instrument’s assignment, so that Dorico knows about the capabilities of that endpoint, and will then be able to perform the necessary switches to access the available playing techniques more or less auomatically.
If no expression map is available, then it’ll be up to you to define the capabilities of the endpoints yourself. You will be able to specify what device, channel, patch, and combination of switch mechanisms should be used when a given instrument plays a particular playing technique.
The hope is that it will then be possible to save this information – i.e. the specific set of virtual instruments loaded into the rack, together with the information about which instruments and playing techniques are accommodated by each endpoint within those instruments – in such a way that it can be applied to other projects, as a kind of playback template.