The client reads the connection settings stored in the NXS files, which by default are in the Documents/NoMachine directory.
Connections can be started from the NoMachine User Interface (double click on any of the available connections listed there), by double clicking on the icon of the .nxs file and from the command line by running the player and passing the session file (e.g. nxplayer  --session <session file>). 

An additional method to feed the client with connection settings may consist of using standard input, by starting the client process with a single dash option on the command line, e.g. :

nxplayer -

In this context, it's up to the application that starts nxplayer to take care of writing all the necessary settings (in the NXS file format) to the standard input.

This new method can be helpful in environments where connection settings cannot be saved, like in a kiosk mode or when settings are created on the fly on read-only file systems.