Why CUPS adds "Auto" option in "InputTray"

[anonymous]

Using CUPS 1.1.21rc1.

CUPS is adding an "Auto" option for "InputSlot".
For PS PPD files, it looks like it does nothing (So use the printer default 
input tray), for foomatic-PPD files, it is using the first input tray option 
in the list. Verified while vi /tmp/foomatic-rip.ps. I were expecting CUPS 
to either put nothing or put the defaultInputSlot there.

Is it a bug?