When you access the physical desktop remotely, you are connecting to its physical display. This functionality is provided by both the free to download NoMachine for Win/Mac/Linux and Raspberry packages, and the NoMachine Enterprise Desktop product.

On Linux, when connecting to the physical desktop which doesn't have an X server running (e.g. it's a headless machine), NoMachine is able to use its own display service (which is an embedded X server) to let users connect seamlessly to a physical desktop running in the background on the remote machine. Pre-requisite is having a desktop environment installed. This ability is enabled by default in NoMachine Free Edition, but can be activated also for the other server types. See https://kb.nomachine.com/AR03P00973 for more details, section 'how to enable the creation of NoMachine display service'. If multiple users are supported and they connect, they will all see the same desktop.

However, if you run Linux and you want multiple users to each see their own desktop and you want to be able to leave those desktops running in the background so that users can reconnect and find everything as they left it, you need a product from the NoMachine Terminal Server family. With these products you can connect to a "NoMachine virtual desktop" to run "virtual desktop sessions". The virtual desktop functionality allows individual multiple Linux desktops to run on the same host. Each user can have their own personal 'virtual' Linux desktop according to how the Administrator has configured it.

The virtual desktop functionality is not available on Windows or Mac.

You can run a 'virtual desktop' session, provided you have one of the following installed on a remote Linux host: Workstation, Small Business Terminal Server, Terminal Server, or Enterprise Terminal Server. Use either NoMachine Free Edition (which includes both server and player components) or Enterprise Client (NoMachine player only) to connect. As long as the virtual desktop functionality is enabled, the connection GUI will allow you to run a virtual desktop, or alternatively a 'Custom session' (virtual single applications on Linux). Connections via web are also supported.

Depending on the type of server product installed, you will be able to run from 4 virtual desktops to an unlimited number. User numbers are unlimited in that you can have as many user name accounts as needed. It is the number of concurrent virtual desktops which is considered. For example, in the case of Workstation, only 4 virtual desktops will be allowed at any given time; with Small Business Terminal Server that number is 10, and with the Terminal/Enterprise Terminal Servers that number is unlimited.

Related information:

Differences between NoMachine and NoMachine Workstation
https://www.nomachine.com/AR10K00702