It's a 1.5 watt zener so to generate 1.5 watts at 47 volts requires a current of 31.9 mA. This is the maximum allowable zener (EDITED) current but, there's a little sting in the tail. If the device is actually a 50 volt zener then this current is of course a little less. Ditto if the tolerance forces it to be a 43 volt zener but this time the current is slightly higher.
This is largely confirmed by the term "Admis. Zener Current" being 30mA. A 1.5 watt 50V zener can only be driven with 30mA. I'm assuming "Admis." means admissible meaning "acceptable" or "valid".
Zeners don't switch off immediately below their zener voltage. In the data sheet that I found (because your link doesn't work for me), it quotes 24V as being the point where 1uA of reverse current flows. Above that voltage, current will increase and below that voltage current will get less.
It's also worth pointing out that if the zener self-heats and takes the local temperature above 60degC then the zener power rating reduces. Here are two graphs, the left graph shows the non-instantanous rise in current as the zener voltage is approached. The right graph shows the maximum power curve just mentioned and ultimately also governs current flow into the device i.e. if you are putting 30mA in and the temperature rises above 60 then you are exceeding the power rating of the device: -