Browsing Chinese (Hong Kong) translation

11.

A string matching the window class (WM_CLASS property)
The window class can be found out by running
'xprop | grep WM_CLASS' and clicking on a window
(use either both parts separated by a space or only the right part).
NOTE: If you specify neither window title nor window class,
then the very first window to appear will be taken;
omitting both options is NOT recommended.