The SDSS-IV Extended Baryon Oscillation Spectroscopic Survey: final
  Emission Line Galaxy Target Selection
  We describe the algorithm used to select the Emission Line Galaxy (ELG)
sample at $z \sim 0.85$ for the extended Baryon Oscillation Spectroscopic
Survey of the Sloan Digital Sky Survey IV, using photometric data from the
DECam Legacy Survey. Our selection is based on a selection box in the $g-r$ vs.
$r-z$ colour-colour space and a cut on the $g$-band magnitude, to favour
galaxies in the desired redshift range with strong [OII] emission. It provides
a target density of 200 deg$^{-2}$ on the North Galactic Cap (NGC) and of 240
deg$^{-2}$ on the South Galactic Cap (SGC), where we use a larger selection box
because of deeper imaging. We demonstrate that this selection passes the eBOSS
requirements in terms of homogeneity. About 50,000 ELGs have been observed
since the observations have started in 2016, September. These roughly match the
expected redshift distribution, though the measured efficiency is slightly
lower than expected. The efficiency can be increased by enlarging the redshift
range and with incoming pipeline improvement. The cosmological forecast based
on these first data predict $\sigma_{D_V}/D_V = 0.023$, in agreement with
previous forecasts. Lastly, we present the stellar population properties of the
ELG SGC sample. Once observations are completed, this sample will be suited to
provide a cosmological analysis at $z \sim 0.85$, and will pave the way for the
next decade of massive spectroscopic cosmological surveys, which heavily rely
on ELGs. The target catalogue over the SGC will be released along with DR14.
