ACR FLC HAD COPIED CLRNC TO MAINTAIN 3500 FT 5 MI W OF DLF VOR; CROSS DLF 4000 FT 023 DEG RADIAL; CLB FL240 EXPECT FL290. CAPT ADVISED FO; PF; TO BE AT OR ABOVE 4000 FT AT DLF. FO QUESTIONED INSTRUCTIONS AND CAPT CHKED WITH DEP CTL WHO ADVISED TO MAINTAIN 4000 FT. GOT TO 4300 FT AND RETURNED TO 4000 FT NO CONFLICT.

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MY FO AND I HAD RECEIVED AN IFR CLRNC FROM DRT TO DFW FROM DEL RIO APCH CTL WHILE ON THE GND IN DRT. THE CLRNC GIVEN WAS AS FOLLOWS: 'CLRED AS FILED EXCEPT ON DEP CLB AND MAINTAIN 350O FT 5 MI W OF DLF VORTAC; CROSS DLF AT 4000 FT; CLB AND MAINTAIN FL240 EXPECT FL290 10 MINS AFTER DEP. AFTER DLF VORTAC TRACK THE DLF 023 DEG RADIAL 43 DME THEN DIRECT ACTON DIRECT DFW.' A CLRNC VOID TIME WAS GIVEN BUT WAS NOT A FACTOR SINCE WE DEPARTED IMMEDIATELY AFTER BEING RELEASED. AFTER TKOF WE MADE A L- HAND DEP CONSISTENT WITH TFC PATTERN. WE CLBED TO 3500 FT AND WAITED FOR 5 DME (DLF VORTAC). WITHIN 5 DME WE COMMENCED OUR CLB TO 4000 FT. I THEN INFORMED THE FO TO BE AT OR ABOVE 4000 FT AT DLF VORTAC; SHE ASKED ME TO CONFIRM BUT INITIATED THE CLB. WHILE CLBING THROUGH 4000 FT I ASKED DRT APCH TO CONFIRM DLF VORTAC 4000 FT OR ABOVE. APCH RESPONDED 'NEGATIVE DLF AT 4000 FT.' AT THIS POINT WE QUICKLY DSNDED TO 4000 FT. DRT APCH THEN STATED 'DSND AND MAINTAIN 4000 FT.' I RESPONDED 'ROGER 4000 FT.' HE DID NOT ASK FOR OUR ALT AND WE DID NOT VOLUNTEER THE INFO. WE HAD HOWEVER SLIGHTLY EXCEEDED THE 300 FT BARRIER. DRT APCH THEN EXPLAINED THAT THE REASON 4000 FT AT DLF VORTAC WAS ASSIGNED WAS BECAUSE WHEN LNDG AT DLF AFB TO THE NW; THEY HAD PUBLISHED ARRS THAT DICTATED ACFT TO MAINTAIN 5000 FT. THERE WAS NEVER; TO MY KNOWLEDGE; ANY CONFLICTING TFC OR ANY JEOPARDY TO THE SAFETY OF THE FLT. I BELIEVE THE INCIDENT AROSE FROM MISUNDERSTANDING OF THE FLT CLRNC. I ROUTINELY FLY TO AND FROM DRT AND OUR CLRNC HAS ALWAYS BEEN 5 MI W OF DLF VORTAC 3500 FT OR ABOVE DLF VORTAC 4000 FT OR ABOVE. ALL OF MY PREVIOUS DEPS FROM DRT HAD BEEN TO THE SE. THIS ONE HAD BEEN TO THE NW SO THE CLRNC WAS DIFFERENT. WE HAD ALSO ALREADY FLOWN APPROX 6.2 HRS THAT DAY AND MENTALLY I WAS NOT AS KEEN AS I SHOULD HAVE BEEN. AT THE END OF THAT FLT (WHICH TERMINATED OUR DUTY DAY) WE HAD FLOWN A TOTAL OF 7.7 HRS.