MD88 CREW HAD AN ALTDEV IN ZTL CLASS A.

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WE WERE FLYING THE LA GRANGE 1 ARR TO ATL. ZTL CLRED US TO DSND TO FL290 WITH PLT DISCRETION TO FL240. CHKED IN WITH NEXT ATL SECTOR AND GOT CLRNC FOR 'CROSS TIROE INTXN AT 14000 FT.' I STARTED DSCNT FROM FL290 AND WAS HANDED OFF TO NEXT SECTOR. WE CHKED IN WITH 'DSNDING TO 14000 FT AT TIROE.' CTLR THEN STATED 'AMENDY OUR CLRNC TO LEVEL AT FL230.' I STOPPED AT FL230. APPROX 2-3 MIN LATER; I DSNDED FROM FL230 FOR 14000 FT AT TIROE; AND FO CALLED THE DSCNT. ZTL RESPONDED 'GO BACK TO FL230.' THE LOWEST ALT I SAW WAS FL225. WE DID GET A TCASII TA ON ANOTHER ACFT AT 1500 FT BELOW US. I REVERSED THE PITCH AND WAS LEVEL AT FL230 IN APPROX 10 SECONDS. NO HDG CHANGES OR OTHER CALLS WERE GIVEN BY ZTL FOR ANY FURTHER SEPARATION. THE REMAINDER OF FLT WAS ROUTINE. RECOMMENDATION: WHEN ATC AMENDS AN ALT DSCNT RESTR (IE; TIROE AT 14000 FT) WITH AN INTERMEDIATE LEVEL OFF (IE; FL230); THEN THE CTLR SHOULD STATE 'AMEND YOUR DSCNT TO LEVEL AT FL230 AND MAINTAIN FL230.' THIS WILL CLRLY DELETE ANY DOUBT ABOUT THE PREVIOUS INSTRUCTIONS TO CROSS TIROE AT 14000 FT.