The "J3" SMD marking code is for a Vishay 18 V zener diode; the BC is for one of two manufacturers whose part numbers end in
C5V6. The 5V6 suggests 5.6 V, and clicking on the datasheet confirms this.
Old timers like me remember the BZY88-
C5V6 through-hole 400 mW zener diode from Philips and Mullard. You can still find them as NOS (New Old Stock, i.e. never used, but not manufactured recently), e.g.
https://www.silicon-ark.co.uk/bzy88-c5v ... by-philips . There are a number of modern components whose part numbers hark back to the old through-hole days, e.g. the 1N4148WS is the SMD equivalent of the very common through-hole 1N4148 general purpose fast diode.
SMD marking codes are short codes (often 2 or three letters and/or digits and occasionally a special character like an asterisk) that code for the actual part number. Of course, there aren't enough short codes to be unique, so sometimes you also have to use the package and sometimes also the general use category to find out what the part is, and sometimes the codes are just not unique, and you have to guess. No need for guessing in these cases. For example, the only two-leaded package that uses the J3 marking code is the Vishay zener diode. Clicking on the handy datasheet link leads to a table of zener voltages, test currents, etc for the various marking codes.
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