The number of VBA keywords you can use in custom functions is smaller than the number you can use in macros. Custom functions are not allowed to do anything other than return a value to a formula in a worksheet or to an expression used in another VBA macro or function. For example, custom functions cannot resize windows, edit a formula in a cell, or change the font, color, or pattern options for the text in a cell. If you include “action” code of this kind in a function procedure, the function returns the #VALUE! error.

The one action a function procedure can do (apart from performing calculations) is display a dialog box. You can use an InputBox statement in a custom function as a means of getting input from the user executing the function. You can use a MsgBox statement as a means of conveying information to the user. You can also use custom dialog boxes—also called UserForms—but that’s a subject beyond the scope of this introduction.