It is in regards to the Satchels.
The Australia Post API does not handle anything over 1 cm for the width of the satchels.
500g L 34 X W 1 X H 20.5
3kg L 39 X W 1 X H 29.5
5kg L 49.5 X W 1 X H 42
To allow your product to be shipped using the Satchels we recommend setting the Width under 1cm.