t, the toolbar object is enabled.
All toolbar objects may also have an
specified. This is called whenever update-toolbar is called. If it returns
t, the toolbar object will be enabled; if it returns
nil, the object will be disabled.
The function enabled-function should not display a dialog or do anything that may cause the system to hang. In general this means interacting with anything outside the Lisp image, including files, databases and so on.
CAPI User Guide and Reference Manual (Windows version) - 25 Feb 2015