This post demystifies definition of system testing given by Black Box Software Testing Foundations course (BBST) created by Rebecca Fiedler, Cem Kaner and James Bach.
System testing focus is on the value of running system.
And here is addition by Glen Mayers (1979), The Art Of Software Testing, p. 110.
System testing is the process of attempting to demonstrate how the program does not meet its objectives.
In system testing we need to know what are system objectives. Those objectives are system values, and represent reason why would somebody use our system in the first place. Let’s take for example Notepad application. One of Notepad objectives is not losing current work while doing text editing. Part of text edit is undo feature. If system loses current work when we do undo action, that means that system test for undo failed, because its objective would not be met.