Updated October 2013; see History
Controls that have a click action associated with them, such as cmd, may define an accelerator character by inserting a "&" in front of the desired character in the display text of the control. For example, if a button displays "Open", you can make the "O" be the accelerator character by setting the label to "&Open." Such an accelerator character allows the control to be "clicked" by the key combination alt+<char> where <char> is the designated accelerator character. (In the example just described, hitting Alt+O would act the same as clicking on the button.)
Accelerator characters will be indicated on the screen by underlining, although the underlines may not initially appear until the user hits the Alt key.
Note that in order for the accelerator to have a practical effect, the control must define some kind of action in its cmd parameter and its ctype flags must include MBF_KBD.
History
2013 October, A-Shell 6.1.1363: AUI_CONTROL refinement: controls without text, such as icons with no labels, can now still have an accelerator key by inserting it at the start of the tooltip string. For example, to use the accelerator key "H":
TOOLTIP$ = "&H Show History"
or:
TOOLTIP$ = ">&H Show History\<menu def>"
Note that the accelerator key will not display in the tooltip, so you may want to include some kind of textual hint, i.e.
TOOLTIP$ = "&H Show History (Alt+H)"