dialog accepts the first character of labels (when in lists such as checkboxes) as a shortcut, and will cycle through matches if the character corresponds to multiple rows.
The feature is mentioned in documentation for the program and its library as abbreviation, e.g., (library):
dlg_button_to_char
Find the first uppercase character in the label, which we may use for an abbreviation. If the label is empty, return -1. If no uppercase character is found, return 0. Otherwise return the uppercase character.
Normally dlg_draw_buttons and dlg_char_to_button use the first uppercase character. However, they keep track of all of the labels and if the first has already been used in another label, they will continue looking for another uppercase character. This function does not have enough information to make that check.
It does not do autocompletion (doing that would require an additional window).