The context group box on the right side of the Monitor Expression dialog shows the current execution point in your program: the file name, line number, type of view, and thread number.
If you have tried to add an expression to a monitor and the debugger issues the error message "Expression validation failed", you may have included out-of-scope variables or constants in your expression. If you are familiar with the structure of your program, the Context group box may help you determine if scoping problems are the cause of the error.