Pass by value sends a COPY of the data stored in the variable you specify, pass by reference sends a direct link to the variable itself. So if you pass a variable by reference and then change the variable inside the block you passed it into, the original variable will be changed. If you simply pass by value, the original variable will not be able to be changed by the block you passed it into but you will get a copy of whatever it contained at the time of the call.
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