C182 HOLDING FOR DEP AT SAF MISUNDERSTOOD CTLR'S INSTRUCTIONS AND TAXIED ON RWY PRIOR TO ATC APPROVAL; RESULTING IN GAR.

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WHILE HOLDING SHORT; THERE WERE 2 OTHER ACFT IN THE PATTERN DOING TOUCH AND GOES; AS WELL AS ARRIVING TFC (2). I CONTINUED TO HOLD SHORT AS INSTRUCTED BY THE TWR. THE EXPERIMENTAL THAT DEPARTED AHEAD OF ME COMPLETED HIS SECOND FULL TRIP AROUND THE PATTERN WHEN THE CTLR THANKED ME FOR MY PATIENCE. HE ALSO TOLD ME THAT THERE WAS A 'SLOT' COMING UP; AND THAT I MUST BE READY WITHOUT DELAY. I UNDERSTAND HIS INSTRUCTION TO POS AND HOLD WHILE THE CITATION CROSSED THE NUMBERS TO LAND. HE CONTINUED TO GIVE INSTRUCTIONS TO OTHER ACFT WITHOUT PAUSE. THEN I TOOK THE POS ON THE RWY WITHOUT A READBACK. AFTER THE CITATION DEPARTED THE RWY; I EXPECTED CLRNC TO TAKE OFF. INSTEAD; THE TWR CLRED ANOTHER ACFT TO LAND. I RADIOED THAT I WAS ON THE NUMBERS; BUT DID NOT RECEIVE A RESPONSE. THE TWR THEN RADIOED A CLRNC TO ANOTHER ACFT; AND I AGAIN RADIOED THAT I WAS ON THE NUMBERS. HE THEN ACKNOWLEDGED MY POS; INSTRUCTED THE LNDG ACFT TO GO AROUND; AND CLRED ME TO TAKE OFF. AFTER I WAS AIRBORNE; HE ADVISED ME THAT MY CLRNC WAS TO CONTINUE TO HOLD SHORT; AND THAT THE ARPT DOES NOT ISSUE POS AND HOLD CLRNCS. HE THEN THANKED ME FOR THE CALL ADVISING OF MY POS. I DO NOT RECALL THE EXACT WORDING OF THE INSTRUCTIONS. AND; PERHAPS THEY WERE 'HOLD YOUR POS' AND NOT 'POS AND HOLD.' BUT; CONTINUING TO HOLD SHORT UNTIL THE CTLR FINISHED HIS OTHER COMS; AND THEN READING BACK THE POS AND HOLD INSTRUCTION WOULD HAVE AVERTED THIS INCURSION.