The window class is the name of the window’s class as it is registered in the operating system, for example #32770 or IEFrame or Afx:400000:F:13508:62:3627. This window class can be found using Spy tool.
The window class can change from one application run to another, and to cover this case, you can truncate the window class and keep only at least one character. For example, this window class:
Afx:4  will trigger for any window class starting with Afx:4

   Notepad window class for the top window:

Notepad window class for the child window where the text is typed: