+/- 6kV ESD performance mentionned on the MC33901/34901 Physical Layers release are the ISO10605:2008, EN 61000-4-2:2008 specifications with IC powered and unpowered, also called "gun test". This is a system level ESD stress applied on pins leaving the module like CAN H and CAN L where energy to sustain at silicon level is higher than HBM, CDM and MM, and using different set up / current shape. Qualification flow of MC33901 and MC34901 also includes the HBM, MM and CDM with industry standard requirement to support handling aspects.
6kV is the basic requirement for IEC-61004-2, with nearly all modern transceivers able to meet this comfortably. Some solutions support up to 12kV, although I assume this is a process side effect rather than a design choice, as the added value is debateable and suppliers aim for the most cost-optimised solution (and ESD protection takes silicon area).
thank you Tony...I didn't realize standard in auto space are that high...in commercial ICs for wierless, broadband, medical imaging etc ESD is typically tested at 2kV HBM and only 500V CDM and MM...Kris