It means for that employee. I presume you are in the Tax edit screen for an employee and selecting which W4 to be the basis for FWT. Since employees are not obligated to update a W4 each year (although they could do so 52 times per year) the warning you see refers just to that employee and that once changed to 2010 you cannot change that employee back to 2019.
Look at the Employees menu for Employee Forms and you can click on each the 2019 and prior, and the 2020 versions - these links are just pdf that will open and you can compare the worksheets which guide an employee on what numbers to put on the form itself.
