what are the variable we can pass in payload Using Soap in MFT Version 12.2.1.3.0(170820.1700.2557)

recently we were trying to change source content folder but we couldn't able to change like target foldername we can change because it has variables but what about source foldername

does it even exist in first place so how many variable we can pass and what are the variables available in option and how can we extract this variable value from WSDL or payload