ALTDEV OCCURS WHEN FO'S PLT RESPONDS TO TCASII RA; THE WRONG WAY.

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FO FLYING ACFT WITH FLT DIRECTOR SYS USING #2 AUTOPLT AND BOTH #1 AND #1 AUTO THROTTLES. TCASII SYS ANNOUNCED 'TFC; TFC;' SCOPE SHOWED WHITE DIAMOND SLIGHTLY L OF OUR ACFT CTRLINE. FO CONCURRED. HIS SCOPE SHOWED SAME INDICATION AND NUMBER. (S TRANSPONDER EQUIPPED ACFT; LEVEL 900 FT BELOW AND L.) I LOOKED OUTSIDE AT APPROX SCOPE POS FOR VISUAL SIGHTING; WHEN TCASII ANNOUNCED 'MONITOR VERT SPD' TWICE. FO ASKED 'SHOULD I DO WHAT IT SAYS?' I REPLIED 'SURE.' NEXT THING I REALIZED; WE WERE DSNDING AND TCASII ANNOUNCED 'CLB; CLB; NOW.' I CHKED ALT AND IT READ 30700 FT. SCOPE SHOWED SOLID WHITE DIAMOND 700 FT BELOW AND L; WHEN FO STARTED CLB BACK TO 31000 FT. VERT SPD INDICATOR SHOWED RED DOTS THROUGH 6000 FT DOWN AND RED DOTS THROUGH 1000 FT CLB WITH 2 GREEN DOTS ABOVE THE 1000 FT CLB. NEXT TCASII ANNOUNCED 'CLR OF CONFLICT.' WE WERE OUT OF VHF RANGE WITH MAZATLAN CTR AND DIDN'T ESTABLISH HIGH FREQ CONTACT WITH SFO RADIO UNTIL SOME TIME LATER. WE DISCUSSED WHAT A 'MONITOR VERT SPD' ANNOUNCEMENT MEANS. (NO ALTDEV AT THAT MOMENT). NEXT TIME I SHALL MAKE A MORE DEFINITE REPLY SUCH AS; HOLD YOUR ALT OR CLB OR DSND AS APPROPRIATE; INSTEAD OF 'SURE.'