RobotStudio event

Is Work Object Coordinate System a fixed coordinate system or a movable one?

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Hi everyone! 
I am a very new person to ABB robot. In my opinion, Wobj coordinate system is fixed. But i'm not sure, and i still wonder that Wobj is fixed in comparison with Base coordinate system or other coordinate system? And when the robot moves (like sliding on a linear motion guide system), will Base coordinates system and Wobj still fixed???
Could anyone help me make it clear or recommend some documents about this problem!
Thank you all in advance!

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