A B757-200; WITH THE R ENG HIGH PRESSURE BLEED VALVE DEFERRED CLOSED PER THE MEL; DIVERTED DUE TO MISINTERP OF MEL NOTE C AND NON COMMITMENT BY MAINT AND DISPATCH TO MAKE A DECISION.

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ON ARR AT THE ACFT; THERE WAS A MAINT CARRYOVER ITEM REGARDING MEL #36-11-02. A DISCUSSION WITH THE MAINT COORDINATOR AND DISPATCHER ENSUED REGARDING THE HIGH PRESSURE SHUTOFF VALVE FOR THE R ENG; WHICH WAS SHOWN AS INOP. THERE WAS ALSO A NOTE C ASSOCIATED WITH THIS ITEM IN THE MEL. THE NOTE STATED; 'DSCNT; APCH AND LNDG ARE NOT MADE IN ICING CONDITIONS WITHIN 5000 FT AGL.' IN TRYING TO ASCERTAIN FROM THE DISPATCHER AND MAINT COORDINATOR CONFIRMATION IF ITEM C APPLIED TO OUR SIT OR NOT BECAUSE WE NEEDED BOTH ENG ANTI-ICE AND AIRFOIL ANTI-ICE; THEY WOULD NOT COMMIT. THEY WERE FOCUSED ON THE 'EXPECTED' ICING LEVEL AND AVOIDED THE ISSUE OF OUR CONCERN WHICH WAS THAT OUR MANUALS (OPS MANUALS) STATE THAT ENG ANTI-ICE MUST BE USED WITH OUTSIDE AIR TEMP BELOW 10 DEGS TAT (50 DEGS F) AND BELOW; AND I KNEW WE WOULD NEED IT BELOW 5000 FT AGL. OUR WHOLE ISSUE WAS IF ITEM C WAS APPLICABLE TO ENG ANTI-ICE. AFTER FURTHER DISCUSSION; WE WERE ASSURED BY MAINT COORDINATOR THAT ENG ANTI-ICE AND AIRFOIL ANTI-ICE WOULD FUNCTION; AND BE IN COMPLIANCE WITH THE MEL. AFTER TKOF FROM ZZZ; ENRTE TO ZZZ1; THE CAPT AND I STARTED TO RESEARCH FURTHER IN THE MEL. WE DECIDED TO SEND A MESSAGE TO THE DISPATCHER TO CLARIFY AGAIN THAT THE USE OF ENG ANTI-ICE WAS NOT LIMITED BY NOTE C IN THE MEL. THE DISPATCHER COULD NOT CONVEY THE ASSURANCE WE NEEDED TO CONTINUE IN KNOWN ICING CONDITIONS. WITHOUT THIS ASSURANCE AND IN THE INTEREST OF SAFETY; WE FELT THE PRUDENT DECISION WAS TO DIVERT AND NOT CONTINUE INTO THE ICING CONDITIONS PRESENT AT ZZZ1. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE NOTE C IN THE MEL WAS MISLEADING AS ENG ANTI-ICING IS SUPPLIED FROM THE LOW PRESSURE COMPRESSOR STAGE AND IS NOT SUPPLIED FROM THE HIGH STAGE BLEED. THE PROB IN THIS SIT WAS DISPATCH AND MAINT WOULD NOT MAKE A DECISION AND IN THE STATE OF THE CARRIER TODAY NO ONE WANTS TO MAKE THE WRONG DECISION.