Difference between ADUM6403 and ADUM6403ARIZ?
Look on the 2nd to the last page of the data sheet - it tells you what the package options are. You have two types basically - one is a wider body than the other.

Any device you choose MUST have the letters following the base part number hence ADUM6403 is not a valid full part number for the device. The options you have are: -


ARWZ
CRWZ
ARIZ
CRIZ


Read the data sheet on the 2nd to last page to understand what they mean.