You select a command, and one of the elements looks like this:
That means that this element must identify one of the following:
If you right-click on the variable element, a selection list will appear that includes the names of all variables, script created new variables, and fields in the currently open datafile (if one is currently open). Selecting from the right-click menu is the only provided method for selecting a variable.
Manual typing a variable name is not permitted, as selecting from the right-click menu insures there are no typos. However, this mandates that you must enter New Variable commands and name the new variable before you can attempt to select that new variable from the right-click menu on a variable element. It also means you must open a datafile in the DataFile Viewer panel before you can select any field from that datafile as a variable choice.
Should you later rename a New Variable, it's usage elsewhere in a script is not auto-corrected. The same is true for a variable referenced as an element of a datafile. Variable selection can be re-selected from the right-click menu at any time.
Once you've replaced the Variable legend, the cell will forever remain the color associated with a Variable as a reminder that this element of the command must be a variable.