A variable in VA can have a value (Set) or not have a value (Not Set). It is the same thing as saying, 'null', except non-programmers don't know what, 'null' is, and generally programmers can make the connection. If you write a value to the log with a token, instead of a blank (or, 'null'), you will see, 'Not Set'. The reason it's not blank is because blank is a valid value and you'll want to see quickly when something is wrong with your script. So, if you have a text variable, 'myVariable', and you don't ever set the value and you do a Write to Log '{TXT:myVariable}', the log will show, 'Not Set' (because you didn't set it to a value (or even blank (empty)). Or, as TheThingIs pointed out, if you set myVariable to a value and then do a '{TXT:vyMaribael}' (misspelled), you'll see, 'Not Set'.