CLSs rather than the current versions to be used. There is an option on the $UNLOAD command to unload a CLS (free the virtual memory it is using, close any FDnames and release any devices or pseudo devices that it has open). Only one CLS is executing at a time, but one CLS of each type may be active and it is possible to switch from one CLS to another without exiting or unloading the original CLS and then to later return to the original CLS and continue working from where one left off. CLSs that have their own sub-commands usually support a STOP command to exit from the CLS, an MTS and/or a RETURN command to return to the calling CLS or MTS command mode, and commands that begin with a dollar-sign ($) are executed as MTS commands with an immediate return to the original CLS. All CLSs except for the USER CLS execute in system mode in problem state. Limited-service state MTS sessions normally operate in "full-service state", but during times of extreme system overload terminal sessions may be placed into "limited-service state" (LSS). The LSS mechanism is manually enabled by the system operator and is normally only used when the hardware system is operating at reduced capacity due to a malfunction. A terminal session is placed into LSS if LSS has been enabled by the system operator and the system is overloaded at signon. LSS sessions may only issue MTS commands and run programs with a short local time limit. Rather than giving all users poor performance, LSS limits the size of the tasks that some users may perform to relatively small tasks such as editing of files and reading of messages in order to allow other users to receive reasonable performance on larger tasks. Users may request that their session be changed to full-service state ($SET LSS=OFF) and such requests are granted if the system is not overloaded at the time the request is made. Command statistics Each MTS command that is issued is recorded, first to a disk file and later to magnetic tape. This information is only available to staff and is used to investigate software problems, security problems, rebate requests, and to provide statistics about how the command language is used. User programs User program refers to a program run by the user and which is not necessarily a program that belongs to