LESS THAN STANDARD SEPARATION BETWEEN ACR AND SMALL TURBOPROP ACFT AT FL200.

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CLBING NE OUT OF ATL; I BELIEVE WE WERE CLRED TO AN INTERMEDIATE ALT OF FL230. THE F/O OBVIOUSLY THOUGHT WE WERE CLRED TO FL230 AS HE HAD SET 230 IN THE MODE CONTROL PANEL (I WAS FLYING). AS WE CLBED THROUGH 20300'; THE CTLR ASKED OUR ALT AND TOLD US WE WERE ASSIGNED FL190 DUE TO TFC; AND TO TURN RIGHT IMMEDIATELY TO A HDG OF 170 DEGS. AS I INITIATED THE TURN I ALSO TOOK ACTION TO RETURN THE AIRPLANE TO FL190. I THEN SAW OUR TFC (SMA Y) PASS FROM RIGHT TO LEFT IN MY SIDE WINDOW. I ESTIMATE WE HAD BEEN AS HIGH AS 20500'; WERE NOW DSNDING INTO THE TFC. APPARENTLY IF WE HAD BEEN CLRED TO A FL190 HOLD DOWN ALT FOR TFC; I MISSED HEARING THE CLRNC. IF THE F/O HAD HEARD AND RESPONDED (ANOTHER FLT COULD HAVE RESPONDED TO OUR FLT); HE MISSED RESETTING THE MODE CONTROL PANEL ALT SELECTOR. WE ALSO HAD AN AUX CREW MEMBER ON THE JUMPSEAT WHO THOUGHT WE WERE CLRED TO FL230. OBVIOUSLY SOMEONE MADE A MISTAKE! COMPOUNDING THAT MISTAKE; MY REACTION WAS TO HEAD FOR THE ALT THE CTLR SAID WE WERE CLRD TO (FL190); WHEN IN FACT WE WERE ALREADY 500' ABOVE THE TFC. UNLESS THE LAG IN ALT RPTING CREATES SUBSTANTIAL DOUBT TO THE CTLR; PERHAPS IT WOULD BE HELPFUL IN HIS RAPID DECISION MAKING FOR THE EVASIVE CLRNC TO INCLUDE A NEW ALT ASSIGNMENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: RPTR (ACR PIC) HAS HAD NO FURTHER WORD AND NO REQUEST FOR STATEMENT FROM FAA. COULD ADD NO DETAILS; BUT SAYS THAT IF CTLR MEANT TO RESTRICT THEIR CLB TO FL190; THEY DID NOT GET THAT RESTRICTION AND DID NOT ACKNOWLEDGE FOR IT. IN RETROSPECT HE SAYS HE SHOULD NOT HAVE STARTED BACK DOWN WITHOUT ASKING CTLR IF THAT IS WHAT HE WANTED. SUPPLEMENTAL INFO FROM ACN 94740: NEXT THE CTLR CALLED OUT TFC TO US AT 2:30 POS AT FL200. I ACKNOWLEDGED THE TFC. WE CONTINUED OUR CLB. WE WERE APPROX 20500' AND CLBING WHEN THE CTLR IN A HURRIED VOICE ASKED US IF WE WERE STILL CLBING. I ACKNOWLEDGED AFFIRMATIVE. HE SAID WE WERE ONLY CLRED TO FL190 AND TO TURN IMMEDIATELY TO A HDG OF 170 DEGS.