I'm not an expert at QFE but according to the (Jeppesen) chart legend, the 0 hPa number refers to the "barometric altitude equivalent".
The way I understand it is such, that if you are using QFE and are receiving at QNH (e.g. from the METAR), subtract the "Rwy Elev" number from the QNH and get the QFE. E.g. at LSZH, with QNH 1013 hPa and Rwy Elev 50 hPa, use a QFE of 1013 - 50 = 963 hPa.
At ZSSS, airport elevation is almost sea level, so QFE and QNH are almost equal, hence Rwy Elev = 0 hPa.
However, I stand to be corrected by an expert.
As for the inset in the ZSSS chart (Jeppesen?), at least the Jeppesen 11-2 chart has a feet @ QNH to meters @ QNH table. China uses metric flight levels and altitudes, so the table is for those (western) airplanes using non-metric altitudes for quick conversion.
550m QNH equals to 1810' MSL. So, if you are asked to descend to 550m (QNH), ATC wants you at 1810' MSL.