On website uml-diagrams.org it is mentioned that state machine diagram is a behavior diagram which shows discrete behavior of a part of designed system through finite state transitions. To the best of my knowledge, state charts represent the states of a single object of a certain class. What is meant by part of the system in this case? Can state diagram represent something other than one object?
Can state diagram represent something other than one object?
Yes. A state diagram shows the states and transitions of a single instance of something. That something can be a class (in which case the instance is called an object), but it can also be a larger structure, like a module, sub-system or even the entire system.
If you take the classic example of an ATM, you can draw a state diagram of an ATM, but in most cases you will further decompose your ATM (sub-)system into multiple classes because it is just too much to handle it all in a single class.