Mitsubishi MELSEC-L series Programming Manual page 46

Indicates the processing performed by the instruction.
Indicates data such as control data, send data or receive data, that are used for an input
argument or output argument in an instruction.
Example: Control data to be used in the CC-Link instruction 'GP_RIRD'
The setting side indicates the following:
User : Data set by user before dedicated instruction execution
System : Data stored by the programmable controller CPU after dedicated instruction
4-4
execution
The setting does not need to be set by the user.
If the setting is set by the user, data cannot be read normally.