A PILOT REPORTS WHILE CLBING ON A SID ATC REQUESTED THEY BE AT OR ABOVE AN ALT THEY WERE ALREADY ABOVE. THE CREW DESCENDED TO THAT ALT RESULTING IN A TCAS RA.

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WE WERE ON THE DEP TO ZZZ; CLBING TO FL230 AT 1500 FPM; APPROX 40 NM S OF ZZZ. AT ABOUT 17000 FT; ATC ASKED US TO EXPEDITE OUR CLB TO FL230. WE INCREASED OUR CLB RATE TO 2000 FPM. AT ABOUT FL187; ATC AGAIN TOLD US TO EXPEDITE CLB AND ADDED 'I NEED YOU AT FL180 IN 30 SECONDS OR LESS.' I RESPONDED THAT WE WERE ALREADY AT FL187. THE CTLR AGAIN SAID 'I NEED YOU AT FL180 IN 30 SECONDS OR LESS.' I ASKED THE FO IF THAT MEANT THE CTLR WANTED US TO GO BACK DOWN TO FL180. HE SAID 'I THINK SO;' AND LEVELED OFF AND THEN BEGAN A DSCNT TO FL180. I QUERIED THE CTLR AS TO WHETHER HE WANTED US BACK DOWN AT FL180. SIMULTANEOUSLY; AT FL185 WE GOT A TCAS TA SHOWING A TARGET AT FL180 ABOUT 10 NM AHEAD OF US; AND THE CTLR SAID 'EXPEDITE CLB TO FL230.' I TOOK CTL OF THE ACFT; DISENGAGED THE AUTOPLT; AND BEGAN AN ABRUPT CLB. THE CTLR THEN ISSUED A HDG OF 270 DEGS. WE BEGAN THE TURN; RE-ENGAGED THE AUTOPLT AND RETURNED THE ACFT TO THE FO. THE TFC ALERT ENDED IMMEDIATELY WHEN WE BEGAN THE CLB. THE CTLR THEN CLRED US DIRECT ZZZ; GAVE US THE FREQ FOR CTR AND THEN SAID 'THANKS FOR YOUR HELP.' FOR US; THE MAIN CONFUSION BEGAN WHEN THE CTLR TOLD US TO BE AT (OR ABOVE?) FL180 IN 30 SECONDS; BUT WE WERE ALREADY SEVERAL HUNDRED FT ABOVE THAT ALT. THEN; WHEN THE CTLR REPEATED THAT SAME INSTRUCTION; IT ADDED TO OUR IMPRESSION THAT HE WANTED US TO GO BACK DOWN TO FL180. BELIEVING THAT HE WANTED US TO DSND; WE BEGAN THAT ACTION WHILE TRYING TO CLARIFY WHAT WE WERE SUPPOSED TO DO. ALSO CONTRIBUTING TO THE CONFUSION; THE FREQ WAS VERY BUSY. THE SAME CTLR WAS WORKING ALL THE AIRLINE TFC ON THE DEP PLUS SEVERAL GA ACFT. WHILE IT IS EASY TO LOOK BACK AND REALIZE THAT WE SHOULD HAVE KEPT CLBING TO FL230; AT THE TIME I BELIEVE WE DID WHAT WE THOUGHT WE SHOULD HAVE DONE. WE UNDERSTOOD WHAT THE CTLR SAID; BUT THE TIMING OF IT MADE IT SEEM LIKE HE WANTED US TO DO SOMETHING ELSE. HOPEFULLY THIS INFO WILL HELP SOMEONE ELSE AVOID THE SAME KIND OF CONFUSION.