B717-200 FLT CREW QUESTION ZLA CTLR'S CLRNC DIRECT TO PGS VORTAC; THEN VIA LAS TYSSN RNAV ARR; IF THE CLRNC WAS ALSO A CLRNC TO 'DSND VIA;' WHICH IT WAS NOT.

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THIS WAS MY FIRST TIME FLYING TO LAS; AND WE WERE CLRED TO PGS; WITH THE PGS 1 ARR. WHEN WE CHKED IN WITH ZLA; WE WERE CLRED DIRECT TO PGS; THEN 'VIA' THE TYSSN RNAV 1 ARR (TYSSN TYSSN1). THE TYSSN ARR IS AN FMS ARR; WITH MANDATORY SPDS AND ALTS; ALL OF WHICH ARE PRELOADED INTO THE FMS. THE ARR IS MUCH LIKE THE KORRY OR DYLAN ARRS AT LGA AND EWR RESPECTIVELY; IN THAT ONCE CLRED FOR THE ARR; YOU SET THE BOTTOM ALT; AND THE FMS WILL FLY A PROFILE DSCNT. HOWEVER; ON BOTH THE KORRY AND DYLAN FMS ARRS; THERE IS A LARGE NOTE THAT YOU MUST MAINTAIN YOUR LAST ASSIGNED ALT UNTIL 'CLRED TO DSND VIA THE ARR.' OUR CLRNC WAS 'CLRED VIA THE ARR' IN LAS. WE CLARIFIED WITH CTR THAT THEY WANTED US TO MAINTAIN FL310; AND FLY THE ARR; WHICH IS WHAT CTR WANTED. THE CTR CTLR ADVISED US THAT THIS IS A COMMON QUESTION; AND THAT THE NEXT CTLR WOULD CLR US TO 'DSND VIA THE ARR.' IF THE TYSSN ARR HAD THE SAME NOTE OF WARNING REGARDING THE NECESSITY TO MAINTAIN YOUR LAST ALT UNTIL SPECIFICALLY CLRED TO 'DSND VIA THE ARR;' I FEEL THIS WOULD BE CLRER. ALSO; THE CTLR PHRASEOLOGY SHOULD INCLUDE A 'MAINTAIN FL310; CLRED PGS AND THE TYSSN ARR.' THAT WOULD ELIMINATE MY CONFUSION.