How can I get the system-information data from the client computer?

The best way to do this is to use CALL CLIENT using the C$SYSINFO library routine.
It returns information about the current operating system.

This library routine produces the same result as the ACCEPT SYSTEM-INFORMATION FROM SYSTEM-INFO statement.
However, in a Thin Client environment, the ACCEPT approach would return the system-information data from the server, while the CALL CLIENT "C$SYSINFO" would return the specific client information.