Hi Andy,
I’ll set out a few basic concepts and nomenclature so you can see the choices you have.
Roon is currently offered in Windows or Mac versions. Linux is coming. All versions are inter-operable, meaning you can have a Core in one OS and Remotes in another. You need a separate Roon licence for each Core.
The first Roon install will usually be the Core. The audio stream always goes through the Core which fetches the input from a library which can include local or network folders. The Core does all the audio processing and these are the hardware requirements. The Core then sends output to various endpoints which are configured as Zones. Zones are either connected to the Core, connected to a Remote (Private Zones) or Network Zones.
You can install Roon on other computers on your network and point them at the Core. They will then be Remotes that configure that Core. A Remote can direct audio to any endpoints connected to the Core and (except for the iOS app) to endpoints connected to it (Private Zones). A Remote can’t currently send audio to Private Zones on other Remotes but that is coming with RoonSpeakers. The audio chain only goes through a Remote if it is directed to a Private Zone on that Remote.
Roon currently supports Meridian and AirPlay Network Zones and any Roon installation can send Output to those Network Zones. The Roon Advanced Audio Transport (RAAT) Software Developer Kit (SDK) has recently been released to the first batch of manufacturers. Further Network Zones will become available as devices are made RoonReady (this generally requires a firmware update). RoonSpeakers will also appear as a Network Zone.
RoonSpeakers will be a separate smaller program with Windows, Mac, Linux, Android and iOS versions. It will turn any device capable of running it into a Network Zone endpoint. Raspberry Pi and Squeezebox devices will also be supported.
A Roon installation can send different outputs to multiple Zones at the same time. You can have one stream to headphones connected to the Core, another to different headphones connected to a Remote, a third to a DAC connected by USB to the Core and a fourth to an AirPlay device in another room. Each of these will have their own Queue and can have Volume control, Volume levelling and Crossfade separately configured.
A Roon installation can send the same output to multiple Zones (called Grouped Zones) provided they are of the same type (eg: Meridian or AirPlay or RoonSpeakers).
Any endpoint recognised by the OS as an audio device on a Roon installation should be available as a Zone in Roon. Varying connections such as USB, S/PDIF or Toslink will show as different Zones. It may be necessary to install drivers in Windows to support USB connections.
RoonServer is identical to Roon, but with the GUI removed. It will always be a Core and is provided to enable headless operation on server type computers. RoonServer is configured with a Remote.
Remote applications are available for Android and iOS. There are hardware requirements as per the above linked post.
My profile (click on my icon) sets out a fairly typical RoonServer/USB/DAC architecture. I use a PC Remote and an iOS Mini 3 to control RoonServer.
Hope the above helps, if you have any particular questions be happy to provide further info.