Sequence where we find LBGs at z ∼ 3.3 sample (Onodera et al. 2016) and fit (dashed line) to the sequence from Kojima et al. (2017) are shown in grey. Two different branches are identified for which the [CII]157.6 μm (centre) and [OIII]88.3 μm (top) lines are strong. The top branch contains galaxies that present a [CII] deficit and have a stronger ionisation parameter, log₁₀U ≲ −2.5 while the bottom branch corresponds to galaxies with a normal-to-extreme PDR emission. The bottom panel shows the same information colour-coded with the electronic density, with the grid of models superimposed showing that we should be able to derive metallicities and ionisation parameters in between the two branches.