Silk Test Classic does not initialize variables for you. So if you have not initialized a variable on entry to a function, it will have the value <unset>. It is better to explicitly give a value to a variable than to trust that another function has already initialized it for you. Also, 4Test does not keep local variables after a function exits. The next time the function is called, the local variables could be uninitialized.
If you are in doubt about whether a variable has a reasonable value at a particular point, set a breakpoint there and click or ViewLocal Variables in the Silk Test Classic menu to check the value of the variable.