NPDU and APDU sizes

A client recently asked us why the Device object  Max_APDU_Length_Supported property has a value of 1476 for his Ethernet 8802-3 based device and where did that number come from?  The standard mentions a maximum NPDU length of 1497.  However when they read our code the value calculates out to 1496.  What is the difference between APDU and NPDU […]