ZDC AMENDS DESCENT ALT; WHICH BECOMES A QUESTION WITH THE CAPT INVOLVING AN ALT OVERSHOOT AND CONFLICT WITH OTHER TFC.

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ON THE LANDR 3 ARRIVAL INTO DEN WE WERE AT FL280 WITH A CLRNC TO CROSS LANDR AT 17000 FT. AT OUR TOP OF DESCENT ATC GAVE US A DESCENT TO FL260. I UNDERSTOOD THAT TO MEAN DESCEND NOW TO FL260 THE ON TO CROSS LANDR AT 17000 FT. THE MCP ALT ALERT WAS NOT RESET TO FL260. THE FO GAVE THE CORRECT CALL OF 'OUT OF FL270 FOR FL260.' I QUESTIONED THAT; STILL THINKING WE WERE CLRED TO 17000 FT AND THAT WE WERE HIGH ON THE DESCENT. APCHING FL260 THE FO SAID 'WE ARE ONLY CLRED TO FL260' AND I BEGAN TO LEVEL. WE WENT THROUGH FL260 DOWN TO FL255 AND THEN BACK TO FL260. WE ASKED THE CTLR TO CONFIRM OUR ALT AND WERE TOLD FL260. MY PERCEPTION WAS WE WERE HIGH ON THE DESCENT AND NEEDED TO GET DOWN. I FOCUSED ON THAT INSTEAD OF THE REAL CLRNC TO FL260. THE ALT ALERT ON THE MCP ALWAYS NEEDS TO BE SET TO YOUR ASSIGNED ALT! SUPPLEMENTAL INFO FROM ACN 526353: THE CAPT RESET THE MCP TO 17000 FT AND ENSURED 17000 FT AND 250 KIAS AT LANDR WERE IN THE FMC. AT OUR TOP OF DESCENT ATC AMENDED OUR ALT TO MAINTAIN FL260. I ACCEPTED THE CLRNC AND READ IT BACK TO ATC. AT THIS TIME I NOTICED THE CAPT INITIATING A 3100 FPM DESCENT FOR OUR 2000 FT ALT CHANGE AND ALL PRIOR TO RESETTING THE MCP ALT. I POINTED TO 17000 FT SET IN THE WINDOW AND TRIED TO CLARIFY THAT THE CLRNC WAS ONLY TO FL260 AND THAT IT SHOULD BE RESET. BY THIS POINT I WAS CALLING 'FL270 FOR FL260.' WE WERE RAPIDLY APCHING FL260 SO I BECAME ADAMANT THAT THE CLRNC WAS FL260. TO THIS THE CAPT REPLIED THAT WE WERE TO EXPEDITE THROUGH FL260 DOWN TO 17000 FT. I PUT MY HANDS ON THE CTLS TO STOP THE DESCENT AND INSISTED THE CLRNC HAD CHANGED. I VERIFIED THE ALT AND WE REGAINED A POSITIVE RATE OF CLIMB BACK TO FL260 BY FL255. THERE WERE NO TCAS ADVISORIES ASSOCIATED WITH THIS EVENT. COMPLETE CREW SITUATIONAL AWARENESS; DOUBLE CHECK MCP PRIOR TO INITIATING ANY ALT CHANGE; ATC MONITORING DESCENTS FOR POSSIBLE SEPARATION CONFLICTS AND VERIFYING WITH FLT CREW.