For a detailed explanation on these directories, please see the following article: https://www.nomachine.com/AR12K00765
In brief, the C-hostname-display-sessionID directories are for managing the connection to the physical desktop or virtual desktop. You will probably have also seen S-hostname-display-sessionID directories; these are related to the client monitor run for each connection.
When you terminate a session, NoMachine’s default behaviour is to delete all old directories from your .nx on the server side unless NoMachine is configured to preserve them, e.g. for debug purposes. When you disconnect your virtual desktop, a C-* and the S-* directory is left in your .nx to be ready for the reconnection.
A correctly terminated session is renamed to T-C-*. When you find a F-C* or a F-S* directory in your .nx , it means that something went wrong and the session or the client monitor has been terminated unexpectedly or incorrectly. You can safely remove all T-C*-*, F-C-* and F-S-* directories.
Be careful not to delete a C-* or a S-* directory as it will cause the sudden termination of the current connection.
To ensure that NoMachine deletes all old directories, edit the node configuration file, namely node.cfg, uncomment and set the SessionLogClean key to:
SessionLogClean 1